mips: define allexec files as soft links

This change is a follow up to r13385. It removes physical copies of the
allexec c-files in none/tests/mips32/ and none/tests/mips64/ and defines
them as soft-links to a common file like other arches do.

Credits to Philippe for noticing it.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13388 a5019735-40e9-0310-863c-91ae7b9d1cf9