MIPS/LD/testsuite: XFAIL DT_TEXTREL map file warning test

MIPS/LD/testsuite: XFAIL DT_TEXTREL map file warning test

Remove:

FAIL: DT_TEXTREL map file warning

introduced with commit 63c1f59d6655 ("readonly_dynrelocs"), coming from
the lack of a "dynamic relocation against `foo' in read-only section
`.rodata'" warning message produced.

The cause of the failure is that for sections which have dynamic
relocations attached the MIPS backend sets SHF_WRITE, in
`mips_elf_create_dynamic_relocation', yielding:

$ readelf -S textrel.so | grep rodata
  [ 8] .rodata           PROGBITS        00000254 000254 000004 00  WA  0   0  1
$

so the section is read/write and therefore there is nothing to warn
about.

This arrangement came from commit 7403cb6305f5 ("PATCH for N32 ABI"),
<https://sourceware.org/ml/binutils/1999-q2/msg00375.html>, i.e. for all
practical purposes it has been there since forever, and therefore it can
be considered a part of the ABI and the test case irrelevant for MIPS
targets.

We don't have a clean way to request UNSUPPORTED result, which would be
the most appropriate here, so just XFAIL the test instead.

	ld/
	* testsuite/ld-elf/shared.exp: XFAIL DT_TEXTREL map file warning
	test for `mips*-*-*'.