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/kernel_copy.rs

@@ -612,6 +612,9 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
     static HAS_SENDFILE: AtomicBool = AtomicBool::new(true);
     static HAS_SPLICE: AtomicBool = AtomicBool::new(true);
 
+    // Android builds use feature level 14, but the libc wrapper for splice is
+    // gated on feature level 21+, so we have to invoke the syscall directly.
+    #[cfg(target_os = "android")]
     syscall! {
         fn splice(
             srcfd: libc::c_int,
@@ -623,6 +626,9 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
         ) -> libc::ssize_t
     }
 
+    #[cfg(target_os = "linux")]
+    use libc::splice;
+
     match mode {
         SpliceMode::Sendfile if !HAS_SENDFILE.load(Ordering::Relaxed) => {
             return CopyResult::Fallback(0);