cmd/link: do not mark resource section as writable

Resources are immutable, and all other linkers set this section to be
read-only and not read-write. Fix this oversight by removing the writable
flag.

Change-Id: Ib441bde6620be2000f1685df1ea7bfaebdbe7860
Reviewed-on: https://go-review.googlesource.com/c/go/+/268258


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Alex Brainman <alex.brainman@gmail.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>