cmd/compiler/internal/gc: remove flag argument from fconv (cleanup)

The fconv flag arguments were 0, FmtSharp, and FmtSharp|FmtSign.
The 0 value was used for binary representation only, which was
readily available via Mpflt.String. Otherwise, FmtSharp was always
passed. FmtSign was used to print the '+' sign in case of a positive
number and only needed for complex number formatting. Instead
implemented cconv and handled it there.

Change-Id: I1f77282f995be9cfda05efb71a0e027836a9da26
Reviewed-on: https://go-review.googlesource.com/136195


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