android: Support: add genrule for VCSRevision.h header

android: Support: add genrule for VCSRevision.h header

Cmake build generates the header, in Android blueprint we need to add
a new "llvm-gen-revision" genrule to launch command script that will
generate "llvm/Support/VCSRevision.h" header.
For simplification the new genrule is added to "llvm-headers" library
which is already used to generate attributes and intrinsics headers.
Mesa python script git_sha1_gen.py is added to external/llvm top path,
with the only changes being the relative git_dir path at line 16 that
is changed from '..' to '.' and the content of generated header.