Since insns are always stored LE, on a BE system the opcodes will be
loaded byte-reversed. Therefore, define two sets of opcodes, one for LE
and one for BE.

This is an NFC for now, as the SME checks for macOS platforms are not
implemented, so zaDisable() is a no-op, but both paths for resuming from
an exception should disable ZA.

This is a fixup for a recent change in #165066.

As it was explained to me in
https://discourse.llvm.org/t/libunwinds-raison-detre/88283/2 the LLVM
version of libunwind is mostly compatible with nongnu one. This change
improves the compatibility a bit further.

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).

For a full toolchain supporting execute-only code generation the runtime
libraries also need to be pre-compiled with it enabled. The generic
`RUNTIMES_EXECUTE_ONLY_CODE` CMake option can now be used during build
configuration to enable execute-only code generation in libunwind.

Related RFC:
https://discourse.llvm.org/t/rfc-execute-only-code-support-for-runtime-libraries-on-aarch64/86180

When originally introduced to libunwind as part of #112171, FEAT_PAuthLR
had its Call Frame Instruction's (CFI's) in a different location to
other Signing Authentication methods. To incorporate this in libunwind,
a 4 byte offset was introduced to work with this. However, this design
was reversed in #121551 so the CFI's are emitted in the same location as
other methods. When making this change, the offset in libunwind was not
removed, so libunwind's PC value would be incorrect.

As the 4 byte offset is no longer needed, that adjustment can be
removed. results->ptrAuthDiversifier will still be set.