[ELF] Add target-specific relocation scanning for x86 (#178846)

Implement scanSection/scanSectionImpl for i386 and x86-64 to

* enable devirtualization of getRelExpr calls
* eliminate abstraction overhead for PLT-to-PCRel optimization, TLS
relocations
* optimize for R_X86_64_PC32 and R_X86_64_PLT32: they consist of 95%
  relocations in `lld/ELF/**/*.o` files.

at the cost of more code.

TLS relocation handling is inlined into scanSectionImpl. Also,

- Remove getTlsGdRelaxSkip
- Replace TLS-optimization-specific expressions:
- R_RELAX_TLS_GD_TO_LE, R_RELAX_TLS_LD_TO_LE, R_RELAX_TLS_IE_TO_LE →
R_TPREL
  - R_RELAX_TLS_GD_TO_IE → R_GOT_PC

This follows the pattern established for MIPS and PPC64 in `#163138`.

getRelExpr is simplified to only handle relocations needed for
relocateNonAlloc and relocateEH.

Average "Scan relocations" time (measured by --time-trace) decreases
from 110078 to 101585.

(
```
ruby -rjson -e 't=[]; 50.times { system("numactl -C 0-7 /t/lld1 -flavor gnu --threads=8 @response.txt -o a.out --time-trace"); t << JSON.parse(File.read("a.out.time-trace"))["traceEvents"].find { |e| e["name"] == "Scan relocations" }["dur"] }; puts "Average: #{t.sum / t.size}"'
```
)

GitOrigin-RevId: 8a3b830c94575a97c40c8b8f4a23f910fab5f0f5