Connect up the xacq_xrel test to the build system.  Also, make the
TSX configure test check for that the assembler can deal with 
xacquire and xrelease prefixes.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13649 a5019735-40e9-0310-863c-91ae7b9d1cf9