syscall: implement setenv/unsetenv in the runtime

This is expected starting with Go 1.20.

I've also applied the same modification to syscall_libc.go so that
setenv is only called in a single place.

src/runtime/env_unix.go

@@ -0,0 +1,57 @@
+//go:build linux || darwin
+
+package runtime
+
+// Update the C environment if cgo is loaded.
+// Called from Go 1.20 and above.
+//
+//go:linkname syscallSetenv syscall.runtimeSetenv
+func syscallSetenv(key, value string) {
+	keydata := cstring(key)
+	valdata := cstring(value)
+	// ignore any errors
+	libc_setenv(&keydata[0], &valdata[0], 1)
+}
+
+// Update the C environment if cgo is loaded.
+// Called from Go 1.20 and above.
+//
+//go:linkname syscallUnsetenv syscall.runtimeUnsetenv
+func syscallUnsetenv(key string) {
+	keydata := cstring(key)
+	// ignore any errors
+	libc_unsetenv(&keydata[0])
+}
+
+// Compatibility with Go 1.19 and below.
+//
+//go:linkname syscall_setenv_c syscall.setenv_c
+func syscall_setenv_c(key string, val string) {
+	syscallSetenv(key, val)
+}
+
+// Compatibility with Go 1.19 and below.
+//
+//go:linkname syscall_unsetenv_c syscall.unsetenv_c
+func syscall_unsetenv_c(key string) {
+	syscallUnsetenv(key)
+}
+
+// cstring converts a Go string to a C string.
+// borrowed from syscall
+func cstring(s string) []byte {
+	data := make([]byte, len(s)+1)
+	copy(data, s)
+	// final byte should be zero from the initial allocation
+	return data
+}
+
+// int setenv(const char *name, const char *val, int replace);
+//
+//export setenv
+func libc_setenv(name *byte, val *byte, replace int32) int32
+
+// int unsetenv(const char *name);
+//
+//export unsetenv
+func libc_unsetenv(name *byte) int32