bootstrap.bash: remove exec wrappers

Without this change, building an Android toolchain fails:

$ CGO_ENABLED=1 GOARCH=arm64 GOOS=android ./bootstrap.bash
...
rmdir: failed to remove 'bin/go_android_arm64_exec': Not a directory

Change-Id: Ibc3b1e2fd24b73a63bd3020ce1e813f2b4496125
Reviewed-on: https://go-review.googlesource.com/c/go/+/170941
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bootstrap.bash

@@ -72,6 +72,7 @@ if [ "$goos" = "$gohostos" -a "$goarch" = "$gohostarch" ]; then
 	# prepare a clean toolchain for others.
 	true
 else
+	rm bin/go_${goos}_${goarch}_exec
 	mv bin/*_*/* bin
 	rmdir bin/*_*
 	rm -rf "pkg/${gohostos}_${gohostarch}" "pkg/tool/${gohostos}_${gohostarch}"