kbuild: gcov: enable -fno-tree-loop-im if supported

Clang does not recognize this compiler option.

Reported-by: Prasad Sodagudi <psodagud@codeaurora.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>