Pass -ffunction-sections/-fdata-sections along to gold-plugin

Summary:
These options need to be passed to the plugin in order to have
an effect on LTO/ThinLTO compiles.

Reviewers: mehdi_amini, pcc

Subscribers: jfb, dschuff, mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D24644

llvm-svn: 284140