mips64: modify configure.ac to check for cavium supported toolchain

Modify configure to check if we can pass -march=octeon or -march=octeon2
flags to the compiler. This part is required as it gives us information how
to compile (i.e. whether to compile) tests specific for Cavium Octeon boards.

Patch by Zahid Anwar, with minor modifications.

Related to Bugzilla issue 326444.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13783 a5019735-40e9-0310-863c-91ae7b9d1cf9