Make backtraces work more reliably.

Make backtraces work more reliably.
* guile.scm: Set up debugging parameters, and enable debugging and
source positions while loading.
(cgen-call-with-debugging, cgen-debugging-stack-start): New
functions.
* read.scm: Don't set debugging parameters here.
(catch-with-backtrace): Function deleted.
(-cgen): Simply note the presence or absence of the -b option.
Pass the flag to cgen-call-with-debugging, so debugging is turned
off here if the user didn't request it, for faster computation.
(cgen): Call cgen-debugging-stack-start here, instead of
catch-with-backtrace.

* Makefile.am (GUILE): Explicitly load guile.scm here, and leave a
trailing -s.
(desc, html, opcodes, sim-arch, sim-cpu, gas-test, sim-test):
Don't write out the trailing -s here.
* Makefile.in: Regenerated.
* cgen-doc.scm, cgen-gas.scm, cgen-stest.scm): Don't load
fixup.scm here; let the caller decide which Scheme's customization
file to preload.
* dev.scm: Load guile.scm, not fixup.scm.
* fixup.scm: Deleted; contents have all moved to guile.scm.
* README: Doc fix.

* guile.scm (debug-write): New function.