util: m5term, fix LDFLAGS, standard make variables

util: m5term, fix LDFLAGS, standard make variables

Enables build systems to provide necessary flags to build m5term.
Useful specially if a different linker is intended to be used.

Change-Id: If7f867cc0965d6ad4627b5421e00a99cc3d64989
Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38256
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>