std: Add syscall7 stub for Linux/MIPS

Some syscalls such as fadvise require an extra argument to comply with
the register pair alignment imposed by the ABI.

Wacky, isn't it?

lib/std/os/linux/mips.zig

@@ -115,6 +115,9 @@ pub fn syscall5(number: SYS, arg1: usize, arg2: usize, arg3: usize, arg4: usize,
     );
 }
 
+// NOTE: The o32 calling convention requires the callee to reserve 16 bytes for
+// the first four arguments even though they're passed in $a0-$a3.
+
 pub fn syscall6(
     number: SYS,
     arg1: usize,
@@ -146,6 +149,40 @@ pub fn syscall6(
     );
 }
 
+pub fn syscall7(
+    number: SYS,
+    arg1: usize,
+    arg2: usize,
+    arg3: usize,
+    arg4: usize,
+    arg5: usize,
+    arg6: usize,
+    arg7: usize,
+) usize {
+    return asm volatile (
+        \\ .set noat
+        \\ subu $sp, $sp, 32
+        \\ sw %[arg5], 16($sp)
+        \\ sw %[arg6], 20($sp)
+        \\ sw %[arg7], 24($sp)
+        \\ syscall
+        \\ addu $sp, $sp, 32
+        \\ blez $7, 1f
+        \\ subu $2, $0, $2
+        \\ 1:
+        : [ret] "={$2}" (-> usize)
+        : [number] "{$2}" (@enumToInt(number)),
+          [arg1] "{$4}" (arg1),
+          [arg2] "{$5}" (arg2),
+          [arg3] "{$6}" (arg3),
+          [arg4] "{$7}" (arg4),
+          [arg5] "r" (arg5),
+          [arg6] "r" (arg6),
+          [arg7] "r" (arg7)
+        : "memory", "cc", "$7"
+    );
+}
+
 /// This matches the libc clone function.
 pub extern fn clone(func: fn (arg: usize) callconv(.C) u8, stack: usize, flags: u32, arg: usize, ptid: *i32, tls: usize, ctid: *i32) usize;