unix: add preadv2 and pwritev2 unix-wide.

Add preadv2 and pwrite2 to linux and Android, making majority of
linux_like based systems have support, to allow using the new flags
introduced in #4452.
Move definitions to linux_like/mod.rs instead of each separate system
alone.

Author: Elie-Kh

libc-test/semver/android.txt

@@ -3753,6 +3753,7 @@ prctl
 pread
 pread64
 preadv
+preadv2
 preadv64
 printf
 prlimit
@@ -3856,6 +3857,7 @@ puts
 pwrite
 pwrite64
 pwritev
+pwritev2
 pwritev64
 qsort
 raise

src/unix/linux_like/linux/gnu/mod.rs

@@ -1029,22 +1029,6 @@ extern "C" {
         dirfd: c_int,
         path: *const c_char,
     ) -> c_int;
-    #[cfg_attr(gnu_file_offset_bits64, link_name = "preadv64v2")]
-    pub fn preadv2(
-        fd: c_int,
-        iov: *const crate::iovec,
-        iovcnt: c_int,
-        offset: off_t,
-        flags: c_int,
-    ) -> ssize_t;
-    #[cfg_attr(gnu_file_offset_bits64, link_name = "pwritev64v2")]
-    pub fn pwritev2(
-        fd: c_int,
-        iov: *const crate::iovec,
-        iovcnt: c_int,
-        offset: off_t,
-        flags: c_int,
-    ) -> ssize_t;
     pub fn preadv64v2(
         fd: c_int,
         iov: *const crate::iovec,

src/unix/linux_like/linux/musl/mod.rs

@@ -780,20 +780,6 @@ extern "C" {
         dirfd: c_int,
         path: *const c_char,
     ) -> c_int;
-    pub fn preadv2(
-        fd: c_int,
-        iov: *const crate::iovec,
-        iovcnt: c_int,
-        offset: off_t,
-        flags: c_int,
-    ) -> ssize_t;
-    pub fn pwritev2(
-        fd: c_int,
-        iov: *const crate::iovec,
-        iovcnt: c_int,
-        offset: off_t,
-        flags: c_int,
-    ) -> ssize_t;
     pub fn getauxval(type_: c_ulong) -> c_ulong;
     pub fn renameat2(
         olddirfd: c_int,

src/unix/linux_like/mod.rs

@@ -2093,6 +2093,22 @@ extern "C" {
         buflen: size_t,
         result: *mut *mut passwd,
     ) -> c_int;
+    #[cfg_attr(gnu_file_offset_bits64, link_name = "preadv64v2")]
+    pub fn preadv2(
+        fd: c_int,
+        iov: *const crate::iovec,
+        iovcnt: c_int,
+        offset: off_t,
+        flags: c_int,
+    ) -> ssize_t;
+    #[cfg_attr(gnu_file_offset_bits64, link_name = "pwritev64v2")]
+    pub fn pwritev2(
+        fd: c_int,
+        iov: *const crate::iovec,
+        iovcnt: c_int,
+        offset: off_t,
+        flags: c_int,
+    ) -> ssize_t;
 }
 
 // LFS64 extensions