[libcxx][libcxxabi][libunwind] Use libgcc on Android

Android doesn't have a libgcc_s and uses libgcc instead, so adjust the
build accordingly. This matches compiler-rt's build setup. libc++abi and
libunwind were already checking for libgcc but in a different context.
This change makes them search only for libgcc on Android now, but the
code to link against libgcc if it were present was already there.

Reviewed By: #libc, #libc_abi, #libunwind, rprichard, srhines

Differential Revision: https://reviews.llvm.org/D78787