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
In mc_translate a NULL guard expression is sometimes passed around
functions to indicate a "true" expression. That caused some confusion
and led people to believe believe, that IRDirty::guard could be NULL.
It cannot.
This confusion was indirectly spotted by coverity's checker who figured
out that IRDirty::guard was sometimes unconditionally dereferenced
and sometimes compared against NULL.
Cleaning this up...


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13250 a5019735-40e9-0310-863c-91ae7b9d1cf9