MIPS/GAS: Correct BFD_RELOC_MIPS16_16_PCREL_S1 fixup size

MIPS/GAS: Correct BFD_RELOC_MIPS16_16_PCREL_S1 fixup size

Correct the size of a BFD_RELOC_MIPS16_16_PCREL_S1 fixup made in
`md_convert_frag', fixing a bug introduced with commit c9775dde3277
("MIPS16: Add R_MIPS16_PC16_S1 branch relocation support)".  Add test
cases to verify that the overflow of this fixup's in-place addend is
still correctly detected.

	gas/
	* config/tc-mips.c (md_convert_frag): Correct
	BFD_RELOC_MIPS16_16_PCREL_S1 fixup size.
	* testsuite/gas/mips/mips16-branch-addend-4.d: New test.
	* testsuite/gas/mips/mips16-branch-addend-5.d: New test.
	* testsuite/gas/mips/mips16-branch-addend-5.l: New stderr
	output.
	* testsuite/gas/mips/mips16-branch-addend-4.s: New test source.
	* testsuite/gas/mips/mips16-branch-addend-5.s: New test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.