eclass/flag-o-matic.eclass: Add missing MIPS CPU errata flags

eclass/flag-o-matic.eclass: Add missing MIPS CPU errata flags

Noticed during a glibc build for MIPS-III ISA that the -mfix-r4000
and -mfix-r4400 gcc flags got stripped off.  These are needed to work
around known CPU errata in R4000 and R4400 CPUs.  In addition, also
add the -mfix-rm7000 option (and it's -mno form) to fix errata in the
PMC RM7000 CPU, and the -mr10k-cache-barrier to control the generation
of cache barriers to work around the side-effects of R10000's
speculative execution capabilities.

Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Reviewed-by: Matt Turner <mattst88@gentoo.org>