[ELF] Use parallel_for_each for writing.

This changes improves performance of lld, when self-hosting lld, when compared
with the bfd linker. BFD linker on average takes 8 seconds in elapsed time.
lld takes 3 seconds elapased time average. Without this change, lld takes ~5
seconds average. The runtime comparisons were done on a release build and
measured by running linking thrice.

lld self-host without the change
----------------------------------
real    0m3.196s
user    0m4.580s
sys     0m0.832s

lld self-host with lld
-----------------------
user    0m3.024s
user    0m3.252s
sys     0m0.796s

time taken to build lld with bfd
--------------------------------
real    0m8.419s
user    0m7.748s
sys     0m0.632s

llvm-svn: 232460