[lld][ELF] Support relocations R_AVR_LO8_LDI_GS/R_AVR_HI8_LDI_GS

Relocations R_AVR_LO8_LDI_GS/R_AVR_HI8_LDI_GS (indirect calls
via function pointers) only cover range 128KiB. They are
equivalent to R_AVR_LO8_LDI_PM/R_AVR_HI8_LDI_PM within this
range.

But for function addresses beyond this range, GNU-ld emits
trampolines. And this patch implements corresponding thunks
for them in lld.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D147364