util: m5ops, optional extra build flags

util: m5ops, optional extra build flags

This increases compilation control for users. Main use case is building
m5ops as part of an image distribution. Specifying a different sysroot
or dynamic linker may be required when the cross toolchain is built as
part of the process.

Change-Id: Icbd3faa92ea6e084fc4a9b2db83129bce73faf21
Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38416
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>