Fix overflow checking for 32-bit pc-relative relocations on x32.

Fix overflow checking for 32-bit pc-relative relocations on x32.

The problem here is that x32 is really using 64-bit addressing,
while pretending to be 32-bit. Even though the object file format
is 32-bit, we need to do the overflow checking with 64-bit
arithmetic (because that's what the hardware will be using).
This patch overrides the pcrela32_check functions in reloc.h
with target-specific versions that do 64-bit checking.

I've also updated the test case to use -Tdata instead of adding
a huge .space directive, to reduce the size of the .o files.

gold/
	PR gold/19567
	* reloc.h (Relocate_functions::Overflow_check): Add comments.
	* x86_64.cc (X86_64_relocate_functions): New class.
	(Target_x86_64::Relocate::relocate): Use the new class.
	* testsuite/Makefile.am (x86_64_overflow_pc32): Add -Tdata option.
	(x32_overflow_pc32): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/x32_overflow_pc32.sh: New script.
	* testsuite/x86_64_overflow_pc32.s: Remove .space directive.