lib/mpi: Eliminate unused umul_ppmm definitions for MIPS

commit b0c091ae04f6746f541b9be91809e1f4f43e9a65 upstream.

Clang errors out when building this macro:

lib/mpi/generic_mpih-mul1.c:37:24: error: invalid use of a cast in a
inline asm context requiring an l-value: remove the cast or build with
-fheinous-gnu-extensions
                umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
                ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/mpi/longlong.h:652:20: note: expanded from macro 'umul_ppmm'
        : "=l" ((USItype)(w0)), \
                ~~~~~~~~~~^~~
lib/mpi/generic_mpih-mul1.c:37:3: error: invalid output constraint '=h'
in asm
                umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb);
                ^
lib/mpi/longlong.h:653:7: note: expanded from macro 'umul_ppmm'
             "=h" ((USItype)(w1)) \
             ^
2 errors generated.

The C version that is used for GCC 4.4 and up works well with clang;
however, it is not currently being used because Clang masks itself
as GCC 4.2.1 for compatibility reasons. As Nick points out, we require
GCC 4.6 and newer in the kernel so we can eliminate all of the
versioning checks and just use the C version of umul_ppmm for all
supported compilers.

Link: https://github.com/ClangBuiltLinux/linux/issues/605


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>