cmd/compile: skip types.Sym for GC mask symbols

For GC mask symbols, we don't need to create types.Sym, just the
LSym.

Change-Id: I285b518cfd60bfaa3202a02b3005a7122daeb338
Reviewed-on: https://go-review.googlesource.com/c/go/+/313512


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>