cmd/internal/gc,ld: use new flag argument syntax

The usage messages for the flags in gc and ld are using the old
flag argument syntax:
   "arg: description using arg"

Update them to the Go 1.5 flag package's syntax:
   "description using arg"

Fixes #10505

Change-Id: Ifa54ff91e1fd644cfc9a3b41e10176eac3654137
Reviewed-on: https://go-review.googlesource.com/9505


Reviewed-by: Rob Pike <r@golang.org>