devel/wasm3: Fix compilation and clean up port

[diizzy]

Summary

Disable -march=native as this overrides settings by the framework and fails on multiple non x86 platforms
Remove GitHub related variables that are already defined
Replace += with = for GH_TUPLE
Remove forced O3 optimization

References:
https://portsfallout.com/port/31163/
https://docs.freebsd.org/en/books/porters-handbook/book/#makefile-master_sites-github-description
Example 21 "Use of USE_GITHUB with Multiple Distribution Files Using GH_TUPLE" in Porters Handbook
https://docs.freebsd.org/en/books/porters-handbook/book/#dads-cflags

Test Plan

Compile tested on FreeBSD 13.0-STABLE #1 stable/13-n247667-dc9094140c9 (amd64) (make, make check-plist)
Poudriere testport OK 12.2-RELEASE (amd64)
Poudriere testport OK 13.0-RELEASE (i386)

[tcberner]

I would split it into two commits

1. COMMENT & pkg-descr
2. build fix

[diizzy]

Sure, I'll wait on maintainer approval and/or timeout first.