Build and enable LLVM's gold plugin.

[rpaulo]

Summary

Build and enable LLVM's gold plugin.

Test Plan

Passed poudriere bulk build with -t.

% cat t.c
int
main() {}
% clang37 -fuse-ld=gold -flto -v t.c
clang version 3.7.0 (tags/RELEASE_370/rc1)
Target: x86_64-unknown-freebsd11.0
Thread model: posix
 "/usr/local/llvm37/bin/clang-3.7" -cc1 -triple x86_64-unknown-freebsd11.0 -flto -emit-llvm-bc -disable-free -main-file-name t.c -mrelocation-model static -mthread-model posix -mdisable-fp-elim -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -v -dwarf-column-info -resource-dir /usr/local/llvm37/bin/../lib/clang/3.7.0 -fdebug-compilation-dir /home/rpaulo -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gnustep -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/t-7c564e.o -x c t.c
clang -cc1 version 3.7.0 based upon LLVM 3.7.0 default target x86_64-unknown-freebsd11.0
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/llvm37/bin/../lib/clang/3.7.0/include
 /usr/include
End of search list.
 "/usr/local/bin/ld.gold" --eh-frame-hdr -dynamic-linker /libexec/ld-elf.so.1 --hash-style=both --enable-new-dtags -o a.out /usr/lib/crt1.o /usr/lib/crti.o /usr/lib/crtbegin.o -L/usr/lib -plugin /usr/local/llvm37/bin/../lib/LLVMgold.so -plugin-opt=mcpu=x86-64 /tmp/t-7c564e.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/crtend.o /usr/lib/crtn.o

[brooks]

Please add the required bits to the awk in the build-plist target at the end of the Makefile to generate pkg-plist. Otherwise this looks fine.