MAINTAINERS: add CLANG/LLVM BUILD SUPPORT info

MAINTAINERS: add CLANG/LLVM BUILD SUPPORT info

Add keyword support so that our mailing list gets cc'ed for clang/llvm
patches.  We're pretty active on our mailing list so far as code review.
There are numerous Googlers like myself that are paid to support
building the Linux kernel with Clang and LLVM.

Link: http://lkml.kernel.org/r/20190620001907.255803-1-ndesaulniers@google.com
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>