build-infra: Generating X11 wrapper offset file is not cross compilable

The GensrcX11Wrappers.gmk creates the offset file by running a tailored C program that performs
a lot of sizeof commands on the X11 structures. This cannot be cross compiled.

Fortunately there are only two offset files, 32 and 64 bit, and they are stable. Thus they
can be committed into the source code tree. A verification step is run when not cross compiling.
This verification step, regenerates the offset file for the build platforms number of bits and
compares the generated file with the committed file.