ANDROID: build.config.common: enable LLVM=1

This moves builds to all use LLVM=1 argument to make, rather than
CC=​clang NM=llvm-nm OBJCOPY=llvm-objcopy ... (see also
https://www.kernel.org/doc/html/latest/kbuild/llvm.html#llvm-utilities

).

Step 3 of aosp/1405387.

Bug: 65987925
Bug: 141693040
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I0975dcad48b6c5344124343606e1c7f85e76e4ee