Support XLD for setting X_LINKER_TYPE and X_LINKER_VERSION.

[bdrewery]

Summary

This is similar to r300350 for bsd.compiler.mk.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon

Test Plan

~/git/freebsd/bin/sh # make -V LINKER_TYPE -V LINKER_VERSION -V X_LINKER_TYPE -V X_LINKER_VERSION
binutils
21750
~/git/freebsd/bin/sh # make -V LINKER_TYPE -V LINKER_VERSION -V X_LINKER_TYPE -V X_LINKER_VERSION XLD=/usr/bin/ld.lld
binutils
21750
lld
40000

[bdrewery]

Fix after rebase

[emaste]

The X_ magic makes a lot more sense when viewed as this change to the basic non-XLD case.

[bdrewery]

In D11309#234209, @emaste wrote:

The X_ magic makes a lot more sense when viewed as this change to the basic non-XLD case.

Yeah, that's why I split it up into 3 pieces. It's really odd and confusing to read.