This commit adds the new RTS option `-xc', which prints the current
cost-centre stack to stderr whenever an exception is raised.  This is
intended to be a debugging tool, to help trace those "Prelude.head:
empty list" errors that are so frustrating to find.

`-xc' is only available for programs compiled with -prof, and you
probably want -auto as well to get useful information.

This is currently experimental; it would be better if it only
displayed the info for *uncaught* exceptions, but this is harder to
implement.

This commit also makes an OFTEL-inspired extension to the RTS option
space.  Since we've almost run out of options, `-x' is now reserved
for `extended' options.  `-xc' is the first of these, but that leaves
'-x[0-9A-Zabd-wyz]' still to be used!  The prefix `-xx' is reserved
for future extension.