selftests: Fix the .S and .S -> .o rules

Both these rules incorrectly use $< (first prerequisite) rather than
$^ (all prerequisites), meaning they don't work if we're using more than
one .S file as input. Switch them to using $^.

They also don't include $(CPPFLAGS) and other variables used in the
default rules, which breaks targets that require those. Fix that by
using the builtin $(COMPILE.S) and $(LINK.S) rules.

Fixes: a8ba798b ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>