cmd/go: fix cgo linking on netbsd

cmd/go: fix cgo linking on netbsd

NetBSD's built-in linker script for 'ld -r' does not provide a
SEARCH_DIR. As a result libgcc.a is not found when -lgcc is used.

Work around this by determining the path to libgcc (by invoking
gcc with the -print-libgcc-file-name option) and explicitly
referencing the resulting library.

R=golang-dev, iant, aram, lucio.dere, minux.ma
CC=golang-dev
https://golang.org/cl/6470044