[macOS Big Sur] libc symbols for both ARM Mac and x86_64 Mac

[p0nce]

Legacy symbols are still needed on macOS 10.16 "Big Sur" when targeting x86_64.
For AArch64 target, there is no need for such legacy symbols and the symbols are the same than on other Darwins.

This commit allows to build druntime and Phobos on Big Sur targetting either arm64 or x86_64.

[dlang-bot]

Thanks for your pull request and interest in making D better, @p0nce! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3226"

[thewilsonator]

Enforce whitespace before opening parenthesis
grep -nrE "\<(for|foreach|foreach_reverse|if|while|switch|catch|version)\(" $(find src -name '*.d') ; test $? -eq 1
src/core/sys/posix/dirent.d:134:        version(AArch64)
src/core/sys/posix/dirent.d:424:    version(AArch64)
src/core/sys/posix/sys/stat.d:2213:        version(AArch64)
src/core/stdc/math.d:1040:        version(AArch64)

[p0nce]

Updated whitespace and squashed.