x86_64: add distinct MCValue representing symbol index in the linker
For PIE targets, we defer getting an address of value until the linker
has allocated all atoms and performed the relocations. In codegen,
we represent this via `MCValue.linker_sym_index` value.
f132f426b9Jakub Konka committed on 2/5/2022, 11:34:24 PMparent4b3b487