Gate __wasilibc_register_preopened_fd on WASI

This API is only available in WASIp1 and as the libc build for WASIp2 is
changing the symbol was recently deleted. This functionality makes sense
for WASIp1, but for WASIp2 it's a fundamentally different set of
syscalls so this is no longer provided by libc.

Author: alexcrichton

src/wasi/mod.rs

@@ -793,6 +793,7 @@ extern "C" {
         timeout: *const timeval,
     ) -> c_int;
 
+    #[cfg(target_env = "p1")]
     pub fn __wasilibc_register_preopened_fd(fd: c_int, path: *const c_char) -> c_int;
     pub fn __wasilibc_fd_renumber(fd: c_int, newfd: c_int) -> c_int;
     pub fn __wasilibc_unlinkat(fd: c_int, path: *const c_char) -> c_int;