PR ld/20995: MIPS: Set GAS flags correctly for tests
    
    Complement commit 9acc85a62eb7 ("Use dynrelro for symbols in relro
    sections too").
    
    	ld/
    	PR ld/20995
    	* testsuite/ld-elf/elf.exp: Set GAS flags correctly for the
    	`mips*-*-*' target and `pr20995' and `pr20995-2' tests.
    
    (cherry picked from commit 6984613a53528c818482c3a531e17a86bac6e7dc)