BaseTools/Source/C/GNUmakefile: Added a condition for handling invalid ARCH values.

If a user has ARCH already set to x86_64, then the makefile would fail.  In the new code, a check is inserted to ensure that ARCH is set to X64 or IA32.  If it's not, then notify the user of the problem detected and exit.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Mike Wade <michael.wade@gmail.com>