BaseTools GCC: add the compiler flags to the linker command line

Now that we invoke GCC as the linker for the GCC toolchain family,
we can pass the CC flags to the linker as well. This is only
required for LTO (which may involve code generation during the link
stage), but does not interfere with non-LTO builds.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>