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.
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