[ELF] - Use multithreading for building .gdb_index.
[ELF] - Use multithreading for building .gdb_index.

Enables multithreaded .gdb_index building.

Differential revision: https://reviews.llvm.org/D36090

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@309688 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld