[clang][ARM] Fix build failure in <arm_acle.h> for __swp (#151354) In commit d5985905ae8e5b2 I introduced a Sema check that prohibits `__builtin_arm_ldrex` and `__builtin_arm_strex` for data sizes not supported by the target architecture version. However, `arm_acle.h` unconditionally uses those builtins with a 32-bit data size. So now including that header will cause a build failure on Armv6-M, or historic architectures like Armv5. To fix it, `arm_acle.h` now queries the compiler-defined `__ARM_FEATURE_LDREX` macro (also fixed recently in commit 34f59d79209268e so that it matches the target architecture). If 32-bit LDREX isn't available it will fall back to the older SWP instruction, or failing that (on Armv6-M), a libcall. While I was modifying the header anyway, I also renamed the local variable `v` inside `__swp` so that it starts with `__`, avoiding any risk of user code having #defined `v`.
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