tools/perf/build: Fix feature-libunwind-debug-frame handling

tools/perf/build: Fix feature-libunwind-debug-frame handling

Set feature-libunwind-debug-frame. We don't want it in
CORE_FEATURE_TESTS because it's not the generic case, but we
need to set it in the !feature-libunwind case.

Also, because x86 distributions typically don't have
dwarf_find_debug_frame() unwinding method:

  test-libunwind-debug-frame.c:(.text+0x31): undefined reference to `_Ux86_64_dwarf_find_debug_frame'

Restrict this new API to ARM for the time being.

With this patch test-all.c works again, so repeat perf builds
are  fast again:

  comet:~/tip> perf stat --null --repeat 5 make -C tools/perf/
  [...]

       0,452899660 seconds time elapsed                                          ( +-  0,11% )

While with before it was:

  comet:~/tip> perf stat --null --repeat 5 make -C tools/perf/
  [...]

       1,674001829 seconds time elapsed                                          ( +-  0,16% )

[ Includes fix to config/feature-checks/Makefile from Will Deacon. ]

Tested-by: Will Deacon <will.deacon@arm.com>
Tested-by: Jean Pihet <jean.pihet@linaro.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-scsoctqzmou3rpkixCHezy9e@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>