Patch: add systemtap-style marker to _Unwind_DebugHook

[Roland McGrath]

As was mentioned, systemtap's sys/sdt.h has wrapper macros that are
source-compatible with dtrace's sys/sdt.h that you can use without any
material difference.  Systemtap also provides a dtrace wrapper script
intended to be compatible with the build-time uses of dtrace, though using
it adds nothing you need for the systemtap probes to be built right.

In versions of systemtap prior to the forthcoming 1.4, the probes produced
some undesireable overhead in PIC code, in the form of some extra dynamic
relocation records in the resultant DSO, adding to the startup cost in the
dynamic linker.  This added startup cost might be judged unacceptable for
the shared libgcc.so, since in practice it's paid by every process in the
system in its initialization.  So you might want either to make compiling
in the probe conditional on a configure option, or to use an autoconf test
that would only accept the most recent versions of sys/sdt.h (and perhaps
also accept dtrace's versions, if you are going to support dtrace too).


Thanks,
Roland