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/fs.rs

@@ -46,8 +46,8 @@ use libc::fstatat64;
 use libc::readdir_r as readdir64_r;
 #[cfg(target_os = "android")]
 use libc::{
-    dirent as dirent64, fstat as fstat64, fstatat as fstatat64, lseek64, lstat as lstat64,
-    open as open64, stat as stat64,
+    dirent as dirent64, fstat as fstat64, fstatat as fstatat64, ftruncate64, lseek64,
+    lstat as lstat64, off64_t, open as open64, stat as stat64,
 };
 #[cfg(not(any(
     target_os = "linux",
@@ -835,16 +835,10 @@ impl File {
     }
 
     pub fn truncate(&self, size: u64) -> io::Result<()> {
-        #[cfg(target_os = "android")]
-        return crate::sys::android::ftruncate64(self.as_raw_fd(), size);
-
-        #[cfg(not(target_os = "android"))]
-        {
-            use crate::convert::TryInto;
-            let size: off64_t =
-                size.try_into().map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;
-            cvt_r(|| unsafe { ftruncate64(self.as_raw_fd(), size) }).map(drop)
-        }
+        use crate::convert::TryInto;
+        let size: off64_t =
+            size.try_into().map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;
+        cvt_r(|| unsafe { ftruncate64(self.as_raw_fd(), size) }).map(drop)
     }
 
     pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
@@ -1154,7 +1148,7 @@ pub fn link(original: &Path, link: &Path) -> io::Result<()> {
         } else if #[cfg(target_os = "macos")] {
             // On MacOS, older versions (<=10.9) lack support for linkat while newer
             // versions have it. We want to use linkat if it is available, so we use weak!
-            // to check. `linkat` is preferable to `link` ecause it gives us a flag to
+            // to check. `linkat` is preferable to `link` because it gives us a flag to
             // specify how symlinks should be handled. We pass 0 as the flags argument,
             // meaning it shouldn't follow symlinks.
             weak!(fn linkat(c_int, *const c_char, c_int, *const c_char, c_int) -> c_int);