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>