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`

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

The existing thrd_once function has been refactored so that the
implementation can be shared between thrd_once and pthread_once
functions.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D134716