[libc] fix HashTable warnings and build problems (#74371)

According to https://lab.llvm.org/buildbot/#/builders/163/builds/48002,
the generic build on HashTable fails with two major issues with
`werror`:
1. warnings on `error: suggest braces around initialization of
subobject`.
2. `__support/HashTable` tests are built regardless of its entrypoints`

This PR attempts to fix such issues.