cmd/compile/internal/gc: better names for (b|c|f)conf (cleanup)

Use String and GoString methods instead of the xconf names
for the numeric conversion routines.

Also, fixed a couple of comments in fmt.go.

Change-Id: I1b8acdd95dbff3fc30273070fbb1ac4860031a3c
Reviewed-on: https://go-review.googlesource.com/136197


Reviewed-by: Matthew Dempsky <mdempsky@google.com>