[ELF] --time-trace: Trace "Write sections"

writeSections is typically a bottleneck.
This was used to track down the following bottlenecks:

* Output section .rela.dyn (9115d751)
* Output section .debug_str (3aae04c7)
* posix_fallocate is slow for Linux tmpfs: D115957

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D115984