lto: leave jobserver fds open for module LDFINAL

When executing a non-parallel, non-make command, make closes its
jobserver fds (make-4.0/job.c +1473), but leaves --jobserver-fds in
$MAKE in the passed environment.  The LTO sub-make crashes when it tries
to read a token from the closed fd.

Signed-off-by: Ryan Pennucci <decimalman@gmail.com>
lto: Switch to -flto=jobserver

Signed-off-by: Ryan Pennucci <decimalman@gmail.com>