Rework the alignment check for BFD_RELOC_MIPS_18_PCREL_S3.

Rework the alignment check for BFD_RELOC_MIPS_18_PCREL_S3.

gas/

	* config/tc-mips.c (md_apply_fix): Apply alignment check
	to the symbol and offset rather than *valP for
	BFD_RELOC_MIPS_18_PCREL_S3.  Also update the error message
	for BFD_RELOC_MIPS_19_PCREL_S2.

gas/testsuite/

	* gas/mips/r6-64.s: Remove .align directives from LDPC
	instructions and add further tests for LDPC.
	* gas/mips/r6-64-n32.d: remove the NOPs from LDPC expected
	output and update for new tests.
	* gas/mips/r6-64-n64.d: Likewise.
	* gas/mips/ldpc-unalign.l: New file.
	* gas/mips/ldpc-unalign.s: Likewise.
	* gas/mips/mips.exp: Run ldpc-unalign test.