mips: Remove compiler check in unroll macro

mips: Remove compiler check in unroll macro

CONFIG_CC_IS_GCC is undefined when Clang is used, which breaks the build
(see our Travis link below).

Clang 8 was chosen as a minimum version for this check because there
were some improvements around __builtin_constant_p in that release. In
reality, MIPS was not even buildable until clang 9 so that check was not
technically necessary. Just remove all compiler checks and just assume
that we have a working compiler.

Fixes: ee7d6ec31b14 ("Restore gcc check in mips asm/unroll.h")
Link: https://travis-ci.com/github/ClangBuiltLinux/continuous-integration/jobs/359642821
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>