BaseTools: Separate HOST and PREFIX env for GCC tool chain

BaseTools: Separate HOST and PREFIX env for GCC tool chain

The crossing GCC compiler may use the different path for make and gcc tool.
So, GCC_HOST_BIN is introduced for make path. GCC5_BIN is still kept for
gcc path. User needs to set GCC_HOST_BIN besides set GCC5_BIN env if
the default make is not used. Normally, make is in the default system path.
GCC_HOST_BIN is not required to be set.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>