boost: fixup mips64 fiber patch

boost: fixup mips64 fiber patch

It turns out that this is also broken on mips64el. Further testing
reveals that

-mips32r2 -mtune=mips32r2 -mabi=32

compiles the PAUSE instruction just fine whereas

-mips64r2 -mtune=mips64r2 -mabi=64

does not. The PAUSE instruction was introduced in version 2.6 of the
MIPS ISA and GCC for some reason does not allow usage of it with MIPS64.
Modify the macro to fix the situation instead of just matching on
octeon, which is not quite correct.

Signed-off-by: Rosen Penev <rosenp@gmail.com>