Fix runtime error: remove -march=native as optimisation target

Details:
The upstream Makefile contains -march=native as optimisation which leads
to unconditional use of AVX instructions if built on a machine that has
AVX support. Subsequently this causes SIGILL on processors without AVX,
including latest-generation Atom descendants.

PR: 225922
Reported by: arthur@qeng-ho.org
Approved by: hsw@bitmark.com (maintainer, implicit)
MFH: 2018Q3