[X86][Darwin] Add Darwin stack probing functions

[aemerson]

Summary

[X86][Darwin] Add Darwin stack probing functions.

The new function is a clone of an existing implementation, and doesn't modify the stack pointer.

rdar://25859140

lib/builtins/CMakeLists.txt

   set(x86_64_SOURCES
       x86_64/chkstk.S
       x86_64/chkstk2.S
+      x86_64/chkstk_darwin.S
       x86_64/floatdidf.c
       x86_64/floatdisf.c
       x86_64/floatdixf.c

lib/builtins/x86_64/chkstk_darwin.S

+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+
+#include "../assembly.h"
+
+#ifdef __x86_64__ 
+
+#define PAGE_SZ 4096
+
+// ___chkstk_darwin is at the moment implemented the same way as ms version.         
+DEFINE_COMPILERRT_FUNCTION(___chkstk_darwin)
+.weak_definition ___chkstk_darwin
+        push   %rcx
+        push   %rax
+        cmp    $PAGE_SZ,%rax
+        lea    24(%rsp),%rcx
+        jb     1f
+2:
+        sub    $PAGE_SZ,%rcx
+        test   %rcx,(%rcx)
+        sub    $PAGE_SZ,%rax
+        cmp    $PAGE_SZ,%rax
+        ja     2b
+1:
+        sub    %rax,%rcx
+        test   %rcx,(%rcx)
+        pop    %rax
+        pop    %rcx
+        ret
+END_COMPILERRT_FUNCTION(___chkstk_darwin)
+
+#endif // __x86_64__