[ELF] Parallelize relocation scanning

* Change `Symbol::flags` to a `std::atomic<uint16_t>`
* Add `llvm::parallel::threadIndex` as a thread-local non-negative integer
* Add `relocsVec` to part.relaDyn and part.relrDyn so that relative relocations can be added without a mutex
* Arbitrarily change -z nocombreloc to move relative relocations to the end. Disable parallelism for deterministic output.

MIPS and PPC64 use global states for relocation scanning. Keep serial scanning.

Speed-up with mimalloc and --threads=8 on an Intel Skylake machine:

* clang (Release): 1.27x as fast
* clang (Debug): 1.06x as fast
* chrome (default): 1.05x as fast
* scylladb (default): 1.04x as fast

Speed-up with glibc malloc and --threads=16 on a ThunderX2 (AArch64):

* clang (Release): 1.31x as fast
* scylladb (default): 1.06x as fast

Reviewed By: andrewng

Differential Revision: https://reviews.llvm.org/D133003