Fix ARM build failure with gcc 4.6.

Gcc option -mno-unaligned-access is supported by gcc 4.7 and newer, so it shouldn't be used with gcc 4.6.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Scott Duplichan <scott@notabs.org>
Reviewed-by: Yingke Liu <yingke.d.liu@intel.com>
Reviewed-by: Olivier Martin <Olivier.martin@arm.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/BaseTools@16464 6f19259b-4bc3-4df7-8a09-765794883524