[lld][ELF] Support relocation R_AVR_LDS_STS_16 on AVRTiny devices

The relocation 'R_AVR_LDS_STS_16' is introduced for the compact
16-bit LDS/STS instructions on AVRTiny devices.

Reviewed By: MaskRay, aykevl

Differential Revision: https://reviews.llvm.org/D139807