Earlier quoted context omitted.
Browser engines like WebKit are (with some work) portable to any platform with a C++ compiler: that's how the current Haiku browser is built.
So, now you just need to build a highly-optimizing, C++ compiler instead of a browser? That's another project that few got right over a period of a decade or so. Of course, one could use a C++ to C compiler or extend existing C++ compiler with OS-specific backend. Might still be plenty work but easier than clean-slate browser.
No. You just use gcc. Like everyone else.