Refactor weak symbols in std::sys::unix

This makes a few changes to the weak symbol macros in `sys::unix`:

- `dlsym!` is added to keep the functionality for runtime `dlsym`
  lookups, like for `__pthread_get_minstack@GLIBC_PRIVATE` that we don't
  want to show up in ELF symbol tables.
- `weak!` now uses `#[linkage = "extern_weak"]` symbols, so its runtime
  behavior is just a simple null check. This is also used by `syscall!`.
  - On non-ELF targets (macos/ios) where that linkage is not known to
    behave, `weak!` is just an alias to `dlsym!` for the old behavior.
- `raw_syscall!` is added to always call `libc::syscall` on linux and
  android, for cases like `clone3` that have no known libc wrapper.

The new `weak!` linkage does mean that you'll get versioned symbols if
you build with a newer glibc, like `WEAK DEFAULT UND statx@GLIBC_2.28`.
This might seem problematic, but old non-weak symbols can tie the build
to new versions too, like `dlsym@GLIBC_2.34` from their recent library
unification. If you build with an old glibc like `dist-x86_64-linux`
does, you'll still get unversioned `WEAK DEFAULT UND statx`, which may
be resolved based on the runtime glibc.

I also found a few functions that don't need to be weak anymore:

- Android can directly use `ftruncate64`, `pread64`, and `pwrite64`, as
  these were added in API 12, and our baseline is API 14.
- Linux can directly use `splice`, added way back in glibc 2.5 and
  similarly old musl. Android only added it in API 21 though.

library/std/src/sys/unix/os/tests.rs

@@ -1,14 +1,12 @@
-use super::*;
-
 #[test]
-#[cfg(not(target_os = "vxworks"))]
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
 fn test_glibc_version() {
     // This mostly just tests that the weak linkage doesn't panic wildly...
-    glibc_version();
+    super::glibc_version();
 }
 
 #[test]
-#[cfg(not(target_os = "vxworks"))]
+#[cfg(all(target_os = "linux", target_env = "gnu"))]
 fn test_parse_glibc_version() {
     let cases = [
         ("0.0", Some((0, 0))),
@@ -20,6 +18,6 @@ fn test_parse_glibc_version() {
         ("foo.1", None),
     ];
     for &(version_str, parsed) in cases.iter() {
-        assert_eq!(parsed, parse_glibc_version(version_str));
+        assert_eq!(parsed, super::parse_glibc_version(version_str));
     }
 }