BaseTools/Build: Consider only build-specified architectures

BaseTools/Build: Consider only build-specified architectures

When building for any specific architecture, the build script today
is loading DSC sections for other architectures not in the build.
The build process should disregard DSC sections that are not
relevant to the build.

This fixes scenario whereby a build occurs in a source tree that was
been cleaned of non-essential directories.  For instance, X64 builds
do not require the ArmPkg directory to build a firmware image.  This
condition (build break when ArmPkg is absent) occurs when included
DSCs have sections for multiple architectures.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Thomas Palmer <thomas.palmer@hpe.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>