cmd/8g: don't call gc.Fatal during initalisation

Fixes #10592

Calling gc.Fatal before gc.Main has been called ends up flushing gc.bstdout before
it is properly set up. Ideally obj.Bflush would handle this case, but that type
and its callers are rather convoluted, so take the simpler route and avoid calling
gc.Fatal altogether.

Change-Id: I338b469e86edba558b6bedff35bb904bfc3d6990
Reviewed-on: https://go-review.googlesource.com/9525


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>