MIPS/ld/testsuite: Limit export class call reloc tests to DSO targets

MIPS/ld/testsuite: Limit export class call reloc tests to DSO targets

Only run MIPS/LD export class call relocation tests with shared library
targets, avoiding:

.../ld/ld-new: -shared not supported

linker errors and removing:

FAIL: MIPS export class call relocation test (o32)

regressions with `mipsel-ps2-elf' and `mips64el-ps2-elf' targets.

	ld/
	* testsuite/ld-mips-elf/mips-elf.exp: Limit export class call
	relocation tests to shared library targets.