cmd/compile: cleanup pragcgo

cmd/compile: cleanup pragcgo

Removes dynimport, dynexport, dynlinker cases since they can not
be reached due to prefix check for "go:cgo_" in getlinepragma.

Replaces the if chains for verb distinction by a switch statement.
Replaces fmt.Sprintf by fmt.Sprintln for string concatenation.

Removes the more, getimpsym and getquoted functions by introducing a
pragmaFields function that partitions a pragma into its components.

Adds tests for cgo pragmas.

Change-Id: I43c7b9550feb3ddccaff7fb02198a3f994444123
Reviewed-on: https://go-review.googlesource.com/21607
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>