Strip my_clang on assignment.

There were a few cases that my_clang was being used without being
stripped. This was causing uses like the following to fail because it
would be partially applied (use clang as the compiler, but don't strip
out incompatible cflags).

    LOCAL_CLANG := true # explanation

To avoid this problem in the future, just strip my_clang when it is
assigned.

Change-Id: I41c2f36a4d4c3aa305a25b4a151c066dad5ffe0f