libitm: Introduce target macro TARGET_BEGIN_TRANSACTION_ATTRIBUTE.

libitm: Introduce target macro TARGET_BEGIN_TRANSACTION_ATTRIBUTE.

The attached patch adds the a target specific attribute via the
new target macro TARGET_BEGIN_TRANSACTION_ATTRIBUTE to the
function begin_transaction().  S/390 uses this to set the
soft-float target attribute which is needed to fix a crash with
-m31.

As there seems to be no place in libitm to document internal macros like
USE_HTM_FASTPATH or the new macro, I've put the documentation in a
comment where the macro is used.

2016-03-03  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	* config/s390/target.h (TARGET_BEGIN_TRANSACTION_ATTRIBUTE): Define
	function attribute to disable floating point in begin_transaction() on
	S/390.
	* beginend.cc (begin_transaction): Use
	TARGET_BEGIN_TRANSACTION_ATTRIBUTE.

From-SVN: r233929