Debugger: Fix return value for PIC functions on x86-64.

- Add Architecture::ResolvePICFunctionAddress() and pull out the
previous code in DwarfImageDebugInfo into the x86-specific
implemementation.

- PIC handling works a bit differently on x86-64 than it does on x86
since x86-64 is likely to use RIP-relative addressing. Implement
ArchitectureX8664::ResolvePICFunctionAddress() accordingly.