dragonfly: add std.c.arc4random_buf

This allows for a more optimal std.crypto.tlcsprng codepath.

Without it a an "incorrect alignment" panic is triggered from
crypto.tlcsprng which aligns a threadlocal but it's actually
not aligned, thus detected by the safety check.

It appears that LLVM-IR does attribute the storage with alignment
but it is ultimately not respected in the final binary for netbsd
and dragonfly.

Author: mikdusan

lib/std/c/dragonfly.zig

@@ -12,6 +12,7 @@ pub extern "c" fn getdents(fd: c_int, buf_ptr: [*]u8, nbytes: usize) usize;
 pub extern "c" fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) c_int;
 pub extern "c" fn getrandom(buf_ptr: [*]u8, buf_len: usize, flags: c_uint) isize;
 pub extern "c" fn pipe2(fds: *[2]fd_t, flags: u32) c_int;
+pub extern "c" fn arc4random_buf(buf: [*]u8, len: usize) void;
 
 pub const dl_iterate_phdr_callback = *const fn (info: *dl_phdr_info, size: usize, data: ?*anyopaque) callconv(.C) c_int;
 pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*anyopaque) c_int;