[runtimes] Synchronize warnings flags between libc++/libc++abi/libunwind

This mostly keeps the same warning flags. The most important exceptions are `-Wpedantic` and `-Wconversion`, which are now removed from libc++abi and libunwind.

Reviewed By: ldionne, #libunwind, #libc, #libc_abi

Spies: mikhail.ramalho, phosek, libcxx-commits

Differential Revision: https://reviews.llvm.org/D144252

GitOrigin-RevId: a7aade1f36eb60161235b66bca46db12e5326a0c