[ELF] Support -z initfirst

Summary:
glibc uses this option to link libpthread.so

glibc/nptl/Makefile:
LDFLAGS-pthread.so = -Wl,--enable-new-dtags,-z,nodelete,-z,initfirst

Reviewers: ruiu, echristo, espindola

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D48329

llvm-svn: 335090