std: only test dlsym! on platforms where it is actually used

`dlsym` doesn't work for finding libc symbols on platforms like linux-musl, so the test will fail.

Author: joboet

library/std/src/sys/pal/unix/weak/tests.rs

@@ -1,6 +1,11 @@
 use super::*;
 
 #[test]
+#[cfg(any(
+    target_vendor = "apple",
+    all(target_os = "linux", target_env = "gnu"),
+    target_os = "freebsd",
+))]
 fn dlsym_existing() {
     const TEST_STRING: &'static CStr = c"Ferris!";
 
@@ -22,6 +27,11 @@ fn dlsym_existing() {
 }
 
 #[test]
+#[cfg(any(
+    target_vendor = "apple",
+    all(target_os = "linux", target_env = "gnu"),
+    target_os = "freebsd",
+))]
 fn dlsym_missing() {
     // Try to find a symbol that definitely does not exist.
     dlsym! {