Auto merge of #49573 - glandium:huge-align, r=SimonSapin

Auto merge of #49573 - glandium:huge-align, r=SimonSapin

Reject huge alignments on macos with system allocator only

ef8804ba277b055fdc3e6d148e680e3c1b597ad8 addressed #30170 by rejecting
huge alignments at the allocator API level, transforming a specific
platform bug/limitation into an enforced API limitation on all
platforms.

This change essentially reverts that commit, and instead makes alloc()
itself return AllocErr::Unsupported when receiving huge alignments.

This was discussed in https://github.com/rust-lang/rust/issues/32838#issuecomment-368348408
and following.