In order to have MVE support, the same bits of the CPACR register that
enable the floating-point extension must be set.

This patch enables the FPU in Arm startup code, which is required to run
tests on Arm configurations with hardware floating-point support.

Summary:
Previous change made us no longer link `exit` by default which implied
the RPC server. This is a required symbol for the loader. This will be
fixed later when I port the loader to LLVMOffload. For now just work
around it to fix the bots.

Summary:
We use the infrastructure to stand up a pretend hosted environment on
the GPU. Part of that is calling exit codes and handling the callback.
Exiting from inside a GPU region is problematic as it actually relies on
a lot of GPU magic behind the scenes. This is at least *correct* now as
we use `quick_exit` on the CPU when the GPU calls `exit`. However,
calling `quick_exit` will interfere with instrumentation or benchmarking
that expects a nice teardown order. For normal execution we should do
the friendly option and let the loader utility clean everything up
manually.

Closes https://github.com/llvm/llvm-project/issues/153666

This patch introduces a new centralized AUXV (auxiliary vector) handling
mechanism for LLVM libc on Linux, replacing the previous scattered
implementation across multiple files.

## Key Changes:

### New Files:
- **libc/src/__support/OSUtil/linux/auxv.h**: New header library
providing
  a clean interface for AUXV access with:
  - `auxv::Entry` struct for AUXV entries (type and value)
  - `auxv::Vector` class with iterator support for traversing AUXV
  - `auxv::get()` function for retrieving specific AUXV values
- Thread-safe initialization with fallback mechanisms (prctl and
/proc/self/auxv)

### Modified Files:

1. **libc/src/__support/OSUtil/linux/CMakeLists.txt**:
   - Added `auxv` header library declaration with proper dependencies:
     - libc.hdr.fcntl_macros
     - libc.src.__support.OSUtil.osutil
     - libc.src.__support.common
     - libc.src.__support.CPP.optional
     - libc.src.__support.threads.callonce

2. **libc/config/linux/app.h**:
   - Removed `AuxEntry` struct (moved to auxv.h as `auxv::Entry`)
   - Removed `auxv_ptr` from `AppProperties` struct
   - Simplified application properties structure

3. **libc/src/sys/auxv/linux/getauxval.cpp**:
   - Completely refactored to use new auxv.h interface
   - Removed ~200 lines of complex initialization code
   - Simplified to just call `auxv::get()` function
- Removed dependencies to external symbols (mman, prctl, fcntl, read,
close, open)

4. **libc/src/sys/auxv/linux/CMakeLists.txt**:
   - Updated dependencies to use new auxv header library
- Removed dependencies to external symbols (prctl, mman, fcntl, unistd,
etc.)

5. **libc/startup/linux/do_start.cpp**:
   - Updated to use new `auxv::Vector` interface
   - Changed from pointer-based to iterator-based AUXV traversal
- Updated field names (`aux_entry->id` → `aux_entry.type`,
`aux_entry->value` → `aux_entry.val`)
- Added call to `auxv::Vector::initialize_unsafe()` for early AUXV setup

6. **libc/startup/linux/CMakeLists.txt**:
   - Added dependency on `libc.src.__support.OSUtil.linux.auxv`

In change #146863 we moved definitions of preinit/init/fini arrays to
header but unintentionally moved outside of the namespace. Since the
namespace also controls the visibility (through LIBC_NAMESPACE_DECL), as
a consequence these symbols no longer have the hidden visibility which
changes the codegen from:

```
 4: 4c11          ldr     r4, [pc, #0x44]         @ 0x4c <__libc_init_array+0x4c>
 6: 4812          ldr     r0, [pc, #0x48]         @ 0x50 <__libc_init_array+0x50>
 8: 447c          add     r4, pc
 a: 4478          add     r0, pc
 c: 1b00          subs    r0, r0, r4
```

to:

```
 4: 4813          ldr     r0, [pc, #0x4c]         @ 0x54 <__libc_init_array+0x54>
 6: 4914          ldr     r1, [pc, #0x50]         @ 0x58 <__libc_init_array+0x58>
 8: 4478          add     r0, pc
 a: 4479          add     r1, pc
 c: 6804          ldr     r4, [r0]
 e: 6808          ldr     r0, [r1]
10: 1b00          subs    r0, r0, r4
```

The `ldr` will trigger a fault in case where these symbols aren't
pointing to a valid memory location which is sometimes the case when the
array is empty.

This is required in hermetic testing downstream. It is not complete, and
will not work on hardware, however it runs on QEMU, and can report a
pass/fail on our tests.

These variants require a different exception table that requires a bit
of initialisation.

This allows us to enable testing for these variants downstream.

Post-commit review changes as suggested by @petrhosek in #146863