bug#74332: zig: tuning-compiler flag translation

[Ekaitz Zarraga]

Hi,

Since 2cc6e09a03c34da832b360fe4e848a9339d9360b we are using a simple 
conversion for -Dcpu and -march that is not correct in every single case.

Gentoo made a translator for CFLAGS and CHOST to Zig flags that we can 
use to make a proper conversion:

https://github.com/gentoo/gentoo/blob/ed835f88bc579f93b2370c0f0569ddbf1ba3f563/eclass/tests/zig-utils.sh