Remove a workaround for an old GCC bug.

This workaround is for GCC 5.4.1. Without this workaround, lld will
produce larger .gdb_index sections for object files compiled with the
buggy version of the compiler.

Since it is not for correctness, and it affects only debug builds (since
you are generating .gdb_index sections), perhaps the hack shouldn't have been
added in the first place. At least, I think it is time to remove this hack.

Differential Revision: https://reviews.llvm.org/D49149

llvm-svn: 336788