kvm-unit-tests: invoke $LD explicitly

Change x86/Makefile.common to invoke the linker directly instead
of using the C compiler as a linker driver.  Plumb LDFLAGS through
to the linker.

This supports building on illumos, allowing the user to use
gold instead of the Solaris linker.  Tested on Linux and illumos.

Signed-off-by: Dan Cross <cross@oxidecomputer.com>
Message-Id: <20220601215749.30223-3-cross@oxidecomputer.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>