[trans-mem] fix begin_transaction ABI on ia32

[trans-mem] fix begin_transaction ABI on ia32

[Aldy Hernandez]

We were calling begin_transaction() with the arguments in the registers,
but the function was expecting them on the stack.

Richard said this was an oversight while converting into C++.

With this patch transactions are now working on ia32, though there are
still 2 distinct failures left on the testsuite.  I will fix these next.

Applying as obvious.

	* libitm.h: Add comment.
	* libitm_i.h (begin_transaction): Add ITM_REGPARM.

Index: libitm.h
===================================================================
--- libitm.h	(revision 161883)
+++ libitm.h	(working copy)
@@ -37,6 +37,7 @@ extern "C" {
 #endif
 
 #ifdef __i386__
+/* Only for 32-bit x86.  */
 # define ITM_REGPARM	__attribute__((regparm(2)))
 #else
 # define ITM_REGPARM
Index: libitm_i.h
===================================================================
--- libitm_i.h	(revision 161512)
+++ libitm_i.h	(working copy)
@@ -244,7 +244,7 @@ struct gtm_transaction
   // Invoked from assembly language, thus the "asm" specifier on
   // the name, avoiding complex name mangling.
   static uint32_t begin_transaction(uint32_t, const gtm_jmpbuf *)
-	__asm__("GTM_begin_transaction");
+	__asm__("GTM_begin_transaction") ITM_REGPARM;
 
   // In eh_cpp.cc
   void revert_cpp_exceptions ();