tools build: Use the same CC for feature detection and actual build

	When build with: 'make CC=clang' we were not using that CC to do
feature detection, which resulted in features being detected with gcc
and then the actual tools being built with clang.

	Most of the time these compilers are compatible enough, so no
problem was being noticed.

	As soon as a system with an old enough clang, one that hasn't
the cpuid.h header is used, and a gcc with it, the "get_cpuid" feature
will be found available but then code that will use can't be compiled.

	Noticed with this combination:

  / $ gcc --version | head -1
  gcc (Alpine 6.3.0) 6.3.0
  / $ clang --version | head -1
  clang version 3.8.1 (tags/RELEASE_381/final)
  / $ cat /etc/alpine-release
  3.5.0
  / $

Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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