This patch reimplements the SME ABI `__arm_za_disable` routine within
libunwind. This routine must be called before resuming from unwinding on
AArch64 platforms with SME support.

Before calling the routine, we need to check that SME is available. In
this patch, this is implemented for Linux-based platforms by checking
HWCAP2. It should be possible to implement this check for other
platforms as required.

This patch includes a test for this functionality. This test requires
SME, so on platforms without it, it will simply pass.

Implementation files using the Intel syntax typically explicitly specify
it. Do the same for the few files where applicable for AT&T.

This enables building LLVM with `-mllvm -x86-asm-syntax=intel` in one's
Clang config files (i.e. a global preference for Intel syntax).

Corrects the spelling of 'IsGlobaLinkage' to 'IsGlobalLinkage' in
XCOFF-related code, comments, and tests across the codebase.

Based on review feedback in #160026.

This makes the substitution a lot more clear now that there is no
documentation around %T.

---------

Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

In the libc++ test suite, we consistently declare main() functions with
full parameters and explicitly return from the function. This helps code
that compiles the tests with -ffreestanding, where main() is not a
special function (with an implicit return and special mangling).

This patch fixes a few stray declarations, including in libunwind and
libc++abi.

Previously, we only used `objcopy`, which is not available for some
build configurations. With this patch, we not only try to use `objcopy`,
but also try to use `llvm-objcopy` if available.

This is a follow-up of https://github.com/llvm/llvm-project/pull/156383.

Our host compiler is a clang install that will default to ld if not told
otherwise.

We were telling meson to use lld, but the way that we did it was
outdated, which lead to picolib producing a linker script that lld could
not use. The tests were in fact linking with ld instead.

Using the `c_ld` setting fixes this problem. See:
https://mesonbuild.com/Machine-files.html#binaries

Then to use lld in tests we need `-fuse-ld=lld` in the config files.

Some of these options were not needed for clang 19.1.7, but were for
clang 21.1.1. We will soon update to 21.1.1 so I have included all of
the required options in this PR.

In https://github.com/llvm/llvm-project/pull/154902, the test failed
with llvm-clang-win-x-aarch64(it is a cross-build, which builds on
Windows and run on Linux, "Win to Aarch64 Linux Ubuntu Cross
Toolchain"), and objdump is not available on Windows(the build env).
Set to require x86 Linux instead.

There is an off-by-one error with current condition check for PC fallen
into the range or not. There is another check within libunwind that use
the correct checks in
https://github.com/llvm/llvm-project/blob/5050da7ba18fc876f80fbeaaca3564d3b4483bb8/libunwind/src/UnwindCursor.hpp#L2757
```
      if ((fdeInfo.pcStart <= pc) && (pc < fdeInfo.pcEnd))
```

The vector granule (AArch64 DWARF register 46) is a pseudo-register that
contains the available size in bits of SVE vector registers in the
current call frame, divided by 64. The vector granule can be used in
DWARF expressions to describe SVE/SME stack frame layouts (e.g., the
location of SVE callee-saves).

The first time VG is evaluated (if not already set), it is initialized
to the result of evaluating a "CNTD" instruction (this assumes SVE is
available).

To support SME, the value of VG can change per call frame; this is
currently handled like any other callee-save and is intended to support
the unwind information implemented in #152283. This limits how VG is
used in the CFI information of functions with "streaming-mode changes"
(mode changes that change the SVE vector length), to make the unwinder's
job easier.