[lld][Hexagon] Fix out-of-range PLT branch thunks (#186545)
Linking large Hexagon binaries (e.g. ASan runtime with >8 MiB of text)
fails with R_HEX_B22_PCREL / R_HEX_PLT_B22_PCREL relocation overflow on
calls to PLT entries, even though the thunk infrastructure exists and
needsThunks is set.
needsThunk() always used s.getVA() to compute the branch destination,
even for PLT calls where the actual destination is the PLT entry. This
meant the distance check used the wrong address and failed to create
thunks when the PLT entry was out of B22_PCREL range.
Fix by using s.getPltVA() when expr == R_PLT_PC. Also override
getThunkSectionSpacing() so ThunkSections are pre-created at appropriate
intervals for large binaries.
