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`.
