[dev.link] cmd/link: use function address directly in pclntab generation
If we are internal linking a static executable, in pclntab
generation, the function addresses are known, so we can just use
them directly instead of emitting relocations.
For external linking or other build modes, we are generating a
relocatable binary so we still need to emit relocations.
Reduce some allocations: for linking cmd/compile,
name old alloc/op new alloc/op delta
Pclntab_GC 38.8MB ± 0% 36.4MB ± 0% -6.19% (p=0.008 n=5+5)