[libc] Enable the '__support' unit tests to be hermetic

This patch updates the `__support` unit tests to be built as hermetic
tests. The only tests we needed to disable were because of the use of
`realloc` which is not implemented correctly as it doesn't copy the
memory.

Depends on D149745

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149750