[MIPS] When calculating a relocation using an undefined weak symbol don't check for...

[MIPS] When calculating a relocation using an undefined weak symbol don't check for overflow.

In MIPS the relocation calculation only ignores the overflow checks for undefined
weak symbols on relocations associated with j/jal.   This patch extends this to
the relocations used by the: b* instructions; pc/gp relative symbol offsets; and the
lwpc/ldpc MIPS r6 instructions.

bfd/
	* elfxx-mips.c (mips_elf_calculate_relocation): Only check for overflow
	on non-weak undefined symbols.

ld/testsuite/
	* ld-mips-elf/mips-elf.exp: Add in undefined weak overflow tests for
	o32, n32 and n64.
	* ld-mips-elf/undefweak-overflow.s: New test.
	* ld-mips-elf/undefweak-overflow.d: New test.
	* ld-mips-elf/undefweak-overflow-n32.d: New test.
	* ld-mips-elf/undefweak-overflow-n64.d: New test.