MIPS: Avoid macro redefinitions

To be able to compile the kernel with LTO, the assembler macros cannot
be declared in the global scope, or the compiler will complain about
redefined macros.

Update the code so that macros are defined then undefined when they are
used.

Note that virt support was added in 2.24 and xpa in 2.25. So we still
need the TOOLCHAIN defines for them.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>