mips: fix link_tool_exe_linux issue for different mips architectures

One issue has been reported on the mailing list by Ilya Smelykh, and the second
issue has been found in development for MIPS64.
The change modifies the way we detect target-arch by reading host_cpu from
config.log rather than asking the toolchain.

Also, for MIPS64, we use:

--section-start=.MIPS.options=$ala

while for o32 we still use:

--section-start=.reginfo=$ala


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13249 a5019735-40e9-0310-863c-91ae7b9d1cf9