Add MIPS Linux support to gcc.misc-tests/linkage.c (testsuite/51748)

This adds MIPS Linux support to gcc.misc-tests/linkage.exp.  Basically
copying what was done for MIPS IRIX and changing the options to be correct.

OK?

gcc/testsuite/ChangeLog:

	PR testsuite/51748
	* gcc.misc-tests/linkage.exp: Add mips*-linux-* support.