Fix restore_rt() getter's inline asm not to clobber the x86-64 red zone

lss/linux_syscall_support.h

 /* Copyright (c) 2005-2011, Google Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 2010 matching lines @@
     }
 
     LSS_INLINE void (*LSS_NAME(restore_rt)(void))(void) {
       /* On x86-64, the kernel does not know how to return from
        * a signal handler. Instead, it relies on user space to provide a
        * restorer function that calls the rt_sigreturn() system call.
        * Unfortunately, we cannot just reference the glibc version of this
        * function, as glibc goes out of its way to make it inaccessible.
        */
       long long res;
-      __asm__ __volatile__("call   2f\n"
-                         "0:.align 16\n"
+      __asm__ __volatile__("jmp    2f\n"
+                           ".align 16\n"
                          "1:movq   %1,%%rax\n"
                            LSS_ENTRYPOINT
-                         "2:popq   %0\n"
-                           "addq   $(1b-0b),%0\n"
-                           : "=a" (res)
+                         "2:leaq   1b(%%rip),%0\n"
+                           : "=r" (res)

[jln (very slow on Chromium), 2013/08/29 20:22:03]

This should always  have been =r, right?

[Mark Seaborn, 2013/08/31 00:01:58]

Yes.  Constraining this to rax was unnecessary.

                            : "i"  (__NR_rt_sigreturn));
       return (void (*)(void))(uintptr_t)res;
     }
   #elif defined(__ARM_ARCH_3__)
     /* Most definitions of _syscallX() neglect to mark "memory" as being
      * clobbered. This causes problems with compilers, that do a better job
      * at optimizing across __asm__ calls.
      * So, we just have to redefine all of the _syscallX() macros.
      */
     #undef LSS_REG
@@ skipping 1615 matching lines @@
 # pragma pop_macro("fstat64")
 # pragma pop_macro("lstat64")
 #endif
 
 #if defined(__cplusplus) && !defined(SYS_CPLUSPLUS)
 }
 #endif
 
 #endif
 #endif