Implement accept4 on x86 android with socketcall syscall.

Linux x86 kernels before 4.3 only support the `socketcall` syscall rather than individual syscalls for socket operations. Since `libc` does a raw syscall for `accept4` on Android, it doesn't work on x86 systems.

This PR instead implements `accept4` for x86 android using `socketcall`. The value for `SYS_ACCEPT4` (in contrast to `SYS_accept4` 👀) is taken from the `linux/net.h` header.

Also note that the `socketcall` syscall takes all arguments as array of long ints. I've double checked with `glibc` to check how they pass arguments, since the Linux man page only says this: "args points to a block containing the actual arguments" and "only standard library implementors and kernel hackers need to know about socketcall()".

This should fix rust-lang/rust#82400

src/unix/linux_like/android/b32/x86/mod.rs

@@ -574,6 +574,35 @@ pub const REG_EFL: ::c_int = 16;
 pub const REG_UESP: ::c_int = 17;
 pub const REG_SS: ::c_int = 18;
 
+// socketcall values from linux/net.h (only the needed ones, and not public)
+const SYS_ACCEPT4: ::c_int = 18;
+
+f! {
+    // Sadly, Android before 5.0 (API level 21), the accept4 syscall is not
+    // exposed by the libc. As work-around, we implement it as raw syscall.
+    // Note that for x86, the `accept4` syscall is not available either,
+    // and we must use the `socketcall` syscall instead.
+    // This workaround can be removed if the minimum Android version is bumped.
+    // When the workaround is removed, `accept4` can be moved back
+    // to `linux_like/mod.rs`
+    pub fn accept4(
+        fd: ::c_int,
+        addr: *mut ::sockaddr,
+        len: *mut ::socklen_t,
+        flg: ::c_int
+    ) -> ::c_int {
+        // Arguments are passed as array of `long int`
+        // (which is big enough on x86 for a pointer).
+        let mut args = [
+            fd as ::c_long,
+            addr as ::c_long,
+            len as ::c_long,
+            flg as ::c_long,
+        ];
+        ::syscall(SYS_socketcall, SYS_ACCEPT4, args[..].as_mut_ptr())
+    }
+}
+
 cfg_if! {
     if #[cfg(libc_align)] {
         mod align;

src/unix/linux_like/android/b64/mod.rs

@@ -305,6 +305,22 @@ pub const UT_LINESIZE: usize = 32;
 pub const UT_NAMESIZE: usize = 32;
 pub const UT_HOSTSIZE: usize = 256;
 
+f! {
+    // Sadly, Android before 5.0 (API level 21), the accept4 syscall is not
+    // exposed by the libc. As work-around, we implement it through `syscall`
+    // directly. This workaround can be removed if the minimum version of
+    // Android is bumped. When the workaround is removed, `accept4` can be
+    // moved back to `linux_like/mod.rs`
+    pub fn accept4(
+        fd: ::c_int,
+        addr: *mut ::sockaddr,
+        len: *mut ::socklen_t,
+        flg: ::c_int
+    ) -> ::c_int {
+        ::syscall(SYS_accept4, fd, addr, len, flg) as ::c_int
+    }
+}
+
 extern "C" {
     pub fn getauxval(type_: ::c_ulong) -> ::c_ulong;
 }

src/unix/linux_like/android/mod.rs

@@ -2350,20 +2350,6 @@ f! {
     pub fn SO_EE_OFFENDER(ee: *const ::sock_extended_err) -> *mut ::sockaddr {
         ee.offset(1) as *mut ::sockaddr
     }
-
-    // Sadly, Android before 5.0 (API level 21), the accept4 syscall is not
-    // exposed by the libc. As work-around, we implement it through `syscall`
-    // directly. This workaround can be removed if the minimum version of
-    // Android is bumped. When the workaround is removed, `accept4` can be
-    // moved back to `linux_like/mod.rs`
-    pub fn accept4(
-        fd: ::c_int,
-        addr: *mut ::sockaddr,
-        len: *mut ::socklen_t,
-        flg: ::c_int
-    ) -> ::c_int {
-        syscall(SYS_accept4, fd, addr, len, flg) as ::c_int
-    }
 }
 
 extern "C" {