[LTO/gold] Support --wrap

Summary:
Utilize new gold plugin api interface for obtaining --wrap option
arguments, and LTO API handling (added for --wrap support in lld LTO),
to mark symbols so that LTO does not optimize them inappropriately.

Note the test cases will be in a new gold test subdirectory that
is dependent on the next release of gold which will contain the new
interfaces.

Reviewers: pcc, tmsriram

Subscribers: mehdi_amini, llvm-commits, inglorion

Differential Revision: https://reviews.llvm.org/D44235

llvm-svn: 327459