kbuild: use -fmacro-prefix-map for .S sources

Follow-up to commit a73619a8 ("kbuild: use -fmacro-prefix-map to
make __FILE__ a relative path"). Assembler sources also use __FILE__
macro so this flag should be also applied to those sources.

Signed-off-by: Denys Zagorui <dzagorui@cisco.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>