Vendor dependencies

Let's see how I like this workflow.

vendor/rustix/src/backend/libc/rand/syscalls.rs

@@ -0,0 +1,16 @@
+//! libc syscalls supporting `rustix::rand`.
+
+#[cfg(target_os = "linux")]
+use {super::super::c, super::super::conv::ret_ssize_t, crate::io, crate::rand::GetRandomFlags};
+
+#[cfg(target_os = "linux")]
+pub(crate) fn getrandom(buf: &mut [u8], flags: GetRandomFlags) -> io::Result<usize> {
+    // `getrandom` wasn't supported in glibc until 2.25.
+    weak_or_syscall! {
+        fn getrandom(buf: *mut c::c_void, buflen: c::size_t, flags: c::c_uint) via SYS_getrandom -> c::ssize_t
+    }
+
+    let nread =
+        unsafe { ret_ssize_t(getrandom(buf.as_mut_ptr().cast(), buf.len(), flags.bits()))? };
+    Ok(nread as usize)
+}