bootstrap.bash: fix build if no exec wrapper exists

Since CL 170941 bootstrap.bash fails if no exec wrapper exists for a
given GOOS/GOARCH yet:

  #### Building ../../go-linux-arm-bootstrap

  Building Go cmd/dist using /usr/local/go.
  Building Go toolchain1 using /usr/local/go.
  Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
  Building Go toolchain2 using go_bootstrap and Go toolchain1.
  Building Go toolchain3 using go_bootstrap and Go toolchain2.
  Building packages and commands for host, linux/amd64.
  Building packages and commands for target, linux/arm.
  rm: cannot remove 'bin/go_linux_arm_exec': No such file or directory

Fix it by using 'rm -f' to ignore nonexisting files.

Change-Id: Ib4b1e19747052fa4dca06319f35cc25a0545c4f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/171722
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>

src/bootstrap.bash

@@ -72,7 +72,7 @@ if [ "$goos" = "$gohostos" -a "$goarch" = "$gohostarch" ]; then
 	# prepare a clean toolchain for others.
 	true
 else
-	rm bin/go_${goos}_${goarch}_exec
+	rm -f bin/go_${goos}_${goarch}_exec
 	mv bin/*_*/* bin
 	rmdir bin/*_*
 	rm -rf "pkg/${gohostos}_${gohostarch}" "pkg/tool/${gohostos}_${gohostarch}"