MIPS: set mips32r5 for virt extensions

Clang's integrated assembler only accepts these instructions when the
cpu is set to mips32r5. With this change, we can assemble
malta_defconfig with Clang via `make LLVM_IAS=1`.

Link: https://github.com/ClangBuiltLinux/linux/issues/763
Reported-by: Dmitry Golovin <dima@golovin.in>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>