[libunwind][Apple] Improve test coverage on Apple platforms (#186423)

Introduces a macro abstraction around capturing the bounds of a
function, which many platforms handle subtly differently (Mach-O, and
ELF, for example).

Also introduce an arm64[^-]* -> aarch64 available feature, to enable
more tests that would otherwise be excluded on Apple platforms, whose
target triples tend to take the form e.g. 'arm64-apple-macosx', rather
than 'aarch64-apple-macosx'.

Third, we implement the has-sme check using the appropriate sysctl, as
getauxval is not available on Darwin platforms.

GitOrigin-RevId: 47e4282c19f095075fda1b5ea67b021f3fa2e3dc