Generating CPU family headers for multiple isas

[Ben Elliston]

   Done analysis.
   Computing sformat argument buffers ...
   ERROR: multiple isas selected (bar baz)
   No backtrace available.

On Frank's suggestion, I reverted to Guile 1.3, which has better (working)
backtraces.  I see that the error arises here:

	[error "multiple isas selected (bar baz)"]
	(if (= (length keep-isa) 1) (current-isa-lookup (car keep-isa)) ...)
	(if (equal? keep-isa (quote #)) (let (#) (if # # ...)) ...)
	(let ((keep-isa #)) (if (equal? keep-isa #) (let # #) ...))
	[current-isa]
	[isa-integral-insn?]
	[#<procedure ()>]
	[force #<promise #<procedure ()>>]
	[adata-integral-insn? #(#("object" # # ...) (# #f # ...))]
	(if (adata-integral-insn? CURRENT-ARCH) 32 ...)
	(let * ((base-length #) (chunk-specs #)) (string-list (...))
	[gen-define-ifields () 0 ...]

This is due to a call in (cgen-cpu.h) to (-gen-extract-macros).

It looks like this function is meant to emit macros for extracting ifields
-- this gives me a bad feeling, since these would need to be emitted for
*each* isa in a multiple-isa cpu.  Any ideas?

Ben