preadv2/pwritev2: Only define these functions with glibc

They're not implemented by musl or bionic.

Author: wmanley

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

@@ -1420,6 +1420,20 @@ extern "C" {
         dirfd: ::c_int,
         path: *const ::c_char,
     ) -> ::c_int;
+    pub fn preadv2(
+        fd: ::c_int,
+        iov: *const ::iovec,
+        iovcnt: ::c_int,
+        offset: ::off_t,
+        flags: ::c_int,
+    ) -> ::ssize_t;
+    pub fn pwritev2(
+        fd: ::c_int,
+        iov: *const ::iovec,
+        iovcnt: ::c_int,
+        offset: ::off_t,
+        flags: ::c_int,
+    ) -> ::ssize_t;
 }
 
 extern "C" {

src/unix/linux_like/linux/mod.rs

@@ -3033,26 +3033,12 @@ extern "C" {
         iovcnt: ::c_int,
         offset: ::off_t,
     ) -> ::ssize_t;
-    pub fn pwritev2(
-        fd: ::c_int,
-        iov: *const ::iovec,
-        iovcnt: ::c_int,
-        offset: ::off_t,
-        flags: ::c_int,
-    ) -> ::ssize_t;
     pub fn preadv(
         fd: ::c_int,
         iov: *const ::iovec,
         iovcnt: ::c_int,
         offset: ::off_t,
     ) -> ::ssize_t;
-    pub fn preadv2(
-        fd: ::c_int,
-        iov: *const ::iovec,
-        iovcnt: ::c_int,
-        offset: ::off_t,
-        flags: ::c_int,
-    ) -> ::ssize_t;
     pub fn quotactl(
         cmd: ::c_int,
         special: *const ::c_char,