Restore gcc check in mips asm/unroll.h

While raising the gcc version requirement to 4.9, the compile-time check
in the unroll macro was accidentally changed from being used on gcc and
clang to being used on clang only.

Restore the gcc check, changing it from "gcc >= 4.7" to "all gcc".

[ We should probably remove this all entirely: if we remove the check
  for CLANG, then the check for GCC can go away. Older versions of clang
  are not really appropriate or supported for kernel builds - Linus ]

Fixes: 6ec4476a

 ("Raise gcc version requirement to 4.9")
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.eti.br>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>