Re-apply "[JITLink][ELF] Add support for ELF::R_X86_64_REX_GOTPCRELX relocation"

This re-applies e2fceec2 with fixes. Apparently we already *do* support
relaxation for ELF, so we need to make sure the test case allocates a slab at
a fixed address, and that the R_X86_64_REX_GOTPCRELX test references an external
that is guaranteed to be out of range.