Fix incorrect x32 overflow checking for refs to weak undef symbols.

Fix incorrect x32 overflow checking for refs to weak undef symbols.

On x32, a pc-relative reference to an undef weak symbol (value 0)
with a negative addend (typically -4) generates a spurious overflow
error because Symbol_value::value() returns a 32-bit negative number
as an unsigned number, which gets zero-extended before subtracting
the PC value. This patch fixes the problem by special-casing the
negative addend, and adding it to the value after widening it to
64 bits. Symbol_value::value() does not need the addend if it's
negative, since it is only important when processing section
symbols for merge sections, where a positive addend provides the
input section offset of the merged constant.

gold/
	* x86_64.cc (X86_64_relocate_functions::pcrela32_check): Fix x32
	overflow checking when symbol value + addend < 0.