rt: Write CFI instructions that (might) work on mac in __morestack

The DW_CFA_val_offset_sf instruction doesn't seem to work on mac,
even after implementing it in the llvm-mc assembler, so now
I'm looking for a different way to communicate the stack pointer
adjustment to the unwinder.

Author: brson

src/rt/arch/x86_64/morestack.S

@@ -71,6 +71,20 @@ MORESTACK:
 	leaq 16(%rbp), %rax
 	pushq %rax
 
+	// During unwinding we want to skip our caller since it's not
+	// a complete frame and will make the unwinder sad
+#if defined(__linux__)
+	// Don't understand this line
+	.cfi_offset 16, 0
+	// Tell the unwinding where to get the stack pointer for
+	// our grandparent frame
+	.cfi_offset %rsp, -24
+#endif
+
+	// Save the grandparent stack pointer for the unwinder
+	leaq 16(%rbp), %rax
+	pushq %rax
+
 	// FIXME: libgcc also saves rax. not sure if we need to
 
 	// Save argument registers