math/gau2grid: remove -march=native, unbreak on i386 and powerpc64

This port added -march=native, which was against ports tree compliance, broke run-time on some amd64 and broke building on non-x86.

I have verified that it builds correctly on i386 and powerpc64 (provided -march=native is removed). It probably also builds on other architectures.

Reviewed by: linimon
MFH: 2020Q1 (fix build blanket)