libc/newlib: Add support for LLVM toolchain

When Zephyr is compiled using LLVM toolchain, we don't need to link with
libgcc to resolve libc dependencies. With this patch, the trick will be
applied only when the GNU compiler is used. Otherwise, we will just link
libc, which works for LLVM.

Signed-off-by: Patryk Duda <pdk@semihalf.com>