Fix ld not respecting --sysroot.

ld(1) uses the /usr/libdata/ldscripts when linking. These scripts add in the
default search paths of /lib and /usr/lib via 'SEARCH_DIR("DIR")'. These
need to be prefixed by '=' so that the --sysroot flag is respected. This
is not a problem with buildworld since the TOOLS_PREFIX is baked into the
cross-ld. However it is a problem when trying to use ld(1) with --sysroot
anywhere else as it ends up still reading /lib and /usr/lib despite --sysroot.

The default --sysroot (TARGET_SYSTEM_ROOT) is '/' for /usr/bin/ld.

I found this while building with META MODE with uses only --sysroot with
/usr/bin/ld, and found that libraries that I had not built in its sysroot
directory were leaking in. This didn't happen with ports binutils either. This
would also impact external compiler support.

Reviewed by: bapt, brooks
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4262