MIPS: Remove GCC_IMM_ASM & GCC_REG_ACCUM macros

asm/compiler.h defined GCC_IMM_ASM & GCC_REG_ACCUM macros, both of which
are defined differently for GCC pre-3.4 or GCC 3.4 & higher. We only
support building with GCC 4.6 & higher since commit cafa0010 ("Raise
the minimum required gcc version to 4.6"), which makes the pre-3.4
definition dead code.

Rather than leave the macro definitions around, inline the GCC 3.4 &
higher definitions into the single file that uses them & remove the
macros entirely.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21000/
Cc: linux-mips@linux-mips.org