Add infrastructural support (IR, VEX) to allow returns of 128-
and 256-bit values from dirty helper functions, in a way which is
independent of the target ABIs and of compilers generating
correct struct return code.

Is a prereq for bug #294285.

MIPS fixes: Petar Jovanovic, mips32r2@gmail.com
S390 fixes: Maran, maranp@linux.vnet.ibm.com



git-svn-id: svn://svn.valgrind.org/vex/trunk@2739 8f6e269a-dfd6-0310-a8e1-e2731360e62c