Don't define away __attribute__ on gcc

Don't define away __attribute__ on gcc

Profile feedback sets -DNO_NORETURN, which causes the compat
header file to go into a default #else block. That #else
block defines away __attribute__(). Doing so causes all
kinds of problems with the Linux and gcc system headers:
in particular it makes the xmmintrin.h headers error out,
breaking the build.

Don't define away __attribute__ when __GNUC__ is set.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>