add std.os.linux.vfork and std.os.linux.exit_group

std/os/linux/index.zig

@@ -719,6 +719,15 @@ pub fn fork() usize {
     return syscall0(SYS_fork);
 }
 
+/// This must be inline, and inline call the syscall function, because if the
+/// child does a return it will clobber the parent's stack.
+/// It is advised to avoid this function and use clone instead, because
+/// the compiler is not aware of how vfork affects control flow and you may
+/// see different results in optimized builds.
+pub inline fn vfork() usize {
+    return @inlineCall(syscall0, SYS_vfork);
+}
+
 pub fn futex_wait(uaddr: usize, futex_op: u32, val: i32, timeout: ?*timespec) usize {
     return syscall4(SYS_futex, uaddr, futex_op, @bitCast(u32, val), @ptrToInt(timeout));
 }
@@ -883,6 +892,11 @@ pub fn exit(status: i32) noreturn {
     unreachable;
 }
 
+pub fn exit_group(status: i32) noreturn {
+    _ = syscall1(SYS_exit_group, @bitCast(usize, isize(status)));
+    unreachable;
+}
+
 pub fn getrandom(buf: [*]u8, count: usize, flags: u32) usize {
     return syscall3(SYS_getrandom, @ptrToInt(buf), count, @intCast(usize, flags));
 }