cmd/compile: update error messages in Mpint, Mpflt methods

cmd/compile: update error messages in Mpint, Mpflt methods

CL 20909 gave Mpint methods nice go-like names, but it
didn't update the names in the error strings. Fix them.

Same for a couple of Mpflt methods.

Change-Id: I9c99653d4b922e32fd5ba18aba768a589a4c7869
Reviewed-on: https://go-review.googlesource.com/30091
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>