Fix ld not respecting --sysroot.

[bdrewery]

Summary

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.

Our default --sysroot (TARGET_SYSTEM_ROOT) is '/'.

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.

[bdrewery]

This modifies these files:

/usr/libdata/ldscripts/elf_x86_64_fbsd.x:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xbn:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xc:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xd:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xdc:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xdw:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xn:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xs:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xsc:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xsw:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
/usr/libdata/ldscripts/elf_x86_64_fbsd.xw:SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");

[bapt]

I went into the same issue while working on making world sysroot friendly!

[brooks]

Looks good.