MIPS/GAS: Handle resolved R6 PC-relative relocations

MIPS/GAS: Handle resolved R6 PC-relative relocations

Complement commit 7361da2c952e ("Add support for MIPS R6.") and fix
internal errors like:

foo.s: Assembler messages:
foo.s: Internal error!
Assertion failure in md_apply_fix at .../gas/config/tc-mips.c:15028.
Please report this bug.

triggered by resolved R6 PC-relative relocations in sources containing
R6 code fragments wrapped into ISA override blocks embedded within code
otherwise assembled for an older ISA.

	gas/
	* config/tc-mips.c (calculate_reloc) <BFD_RELOC_HI16_S_PCREL>
	<BFD_RELOC_LO16_PCREL>: New switch cases.
	(md_apply_fix) <BFD_RELOC_HI16_S_PCREL, BFD_RELOC_LO16_PCREL>:
	Move switch cases along `BFD_RELOC_MIPS_JMP'.
	<BFD_RELOC_MIPS_21_PCREL_S2, BFD_RELOC_MIPS_26_PCREL_S2>
	<BFD_RELOC_MIPS_18_PCREL_S3, BFD_RELOC_MIPS_19_PCREL_S2>: Handle
	the resolved case.
	* testsuite/gas/mips/pcrel-reloc-4.d: New test.
	* testsuite/gas/mips/pcrel-reloc-4-r6.d: New test.
	* testsuite/gas/mips/pcrel-reloc-5.d: New test.
	* testsuite/gas/mips/pcrel-reloc-5-r6.d: New test.
	* testsuite/gas/mips/pcrel-reloc-6.d: New test.
	* testsuite/gas/mips/pcrel-reloc-6.l: New list test.
	* testsuite/gas/mips/pcrel-reloc-4.s: New test source.
	* testsuite/gas/mips/pcrel-reloc-6.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.