kselftest/arm64: Don't pass headers to the compiler as source

kselftest/arm64: Don't pass headers to the compiler as source

The signal Makefile rules pass all the dependencies for each executable,
including headers, to the compiler which GCC is happy enough with but
clang rejects:

   clang --target=aarch64-none-linux-gnu -fintegrated-as -Wall -O2 -g -I/home/broonie/git/linux/tools/testing/selftests/ -isystem /home/broonie/git/linux/usr/include -D_GNU_SOURCE -std=gnu99 -I.  test_signals.c test_signals_utils.c testcases/testcases.c signals.S testcases/fake_sigreturn_bad_magic.c test_signals.h test_signals_utils.h testcases/testcases.h -o testcases/fake_sigreturn_bad_magic
  clang: error: cannot specify -o when generating multiple output files

This happens because clang gets confused about what to do with the
header files, failing to identify them as source.  This is not amazing
behaviour on clang's part and should ideally be fixed but even if that
happens we'd still need a new clang release so let's instead rework the
Makefile so we use variables for the lists of header and source files,
allowing us to only pass the source files to the compiler and keep clang
happy.

As a bonus the resulting Makefile is a bit easier to read.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230111-arm64-kselftest-clang-v1-3-89c69d377727@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>