cmake: host-gcc: Detect libgcc for all archs

This commit updates the build system to always detect the libgcc path
for the host platforms regardless of the target architecture.

The native_posix_64 target previously used the x32 ABI (`-mx32`) and
the multilib for this ABI was not commonly available in the x86-64 host
toolchains.

That is no longer the case because native_posix_64 had been updated to
use the x86-64 ABI (`-m64`).

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>