mips: clean-up in hardware detection

Follow up to VEX r2764.
Add detection of Cavium in /proc/cpuinfo, and test for DSP ASEs only for
platforms with Company ID == MIPS.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13554 a5019735-40e9-0310-863c-91ae7b9d1cf9