Makefile: Set retpoline cflags based on CONFIG_CC_IS_{CLANG,GCC}

This was done as part of commit 7d73c3e9
"Makefile: remove stale cc-option checks" upstream, and is needed to
support backporting further retpoline changes.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>