Bug 444242 - s390x: Sign-extend "relative long" offset in EXRL

Bug 444242 - s390x: Sign-extend "relative long" offset in EXRL

In s390_irgen_EXRL, the offset is zero-extended instead of sign-extended,
typically causing Valgrind to crash when a negative offset occurs.

Fix this with a new helper function that calculates a "relative long"
address from a 32-bit offset.  Replace other calculations of "relative
long" addresses by invocations of this function as well.  And for
consistency, do the same with "relative" (short) addresses.