MIPS/LD/testsuite: Limit reloc 6 tests to DSO targets

MIPS/LD/testsuite: Limit reloc 6 tests to DSO targets

Complement commit 500c05ddbe98 ("MIPS/ld/testsuite: Limit export class
call reloc tests to DSO targets") and also check for target's shared
library support in determining whether to run reloc 6 tests, which
require it.  No changes in results as all targets currently enabled for
NewABI testing support shared libraries.

	ld/
	* testsuite/ld-mips-elf/mips-elf.exp: Limit reloc 6 tests to
	shared library targets.