argon2: don't test if opt.c can build with -march=native.

Avoid a situation where a build host can use SSE2, but opt.c is then
compiled without SSE2 support (which fails).

Also check for CFLAGS for this compile test while here.

amend comment: we avoid -march=native not because of netbsd, but because
it results in shiny package builders creating packages not usable by
some users with older machines.

PR pkg/54238: security/argon2 build fails on i386-current (8.99.41)

(maya)