runtime: copy env strings on startup

Some versions of libc, in this case Android's bionic, point environ
directly at the envp memory.

https://android.googlesource.com/platform/bionic/+/master/libc/bionic/libc_init_common.cpp#104

The Go runtime does something surprisingly similar, building the
runtime's envs []string using gostringnocopy. Both libc and the Go
runtime reusing memory interacts badly. When syscall.Setenv uses cgo
to call setenv(3), C modifies the underlying memory of a Go string.

This manifests on android/arm. With GOROOT=/data/local/tmp, a
runtime test calls syscall.Setenv("/os"), resulting in
runtime.GOROOT()=="/os\x00a/local/tmp/goroot".

Avoid this by copying environment string memory into Go.

Covered by runtime.TestFixedGOROOT on android/arm.

Change-Id: Id0cf9553969f587addd462f2239dafca1cf371fa
Reviewed-on: https://go-review.googlesource.com/7663Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/runtime1.go

@@ -80,7 +80,7 @@ func goenvs_unix() {
 
 	envs = make([]string, n)
 	for i := int32(0); i < n; i++ {
-		envs[i] = gostringnocopy(argv_index(argv, argc+1+i))
+		envs[i] = gostring(argv_index(argv, argc+1+i))
 	}
 }