Bump wasi-libc to SDK 20

The version 17 SDK adds `getpagesize`, so use it instead of hardcoding a
number (even if their implementation is _also_ a hardcoded number.)

src/syscall/syscall_libc_wasi.go

@@ -398,8 +398,7 @@ func Chmod(path string, mode uint32) (err error) {
 }
 
 func Getpagesize() int {
-	// per upstream
-	return 65536
+	return libc_getpagesize()
 }
 
 type Utsname struct {
@@ -428,6 +427,11 @@ func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
 // This is a stub, it is not functional.
 func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 
+// int getpagesize(void);
+//
+//export getpagesize
+func libc_getpagesize() int
+
 // int stat(const char *path, struct stat * buf);
 //
 //export stat