sim/opcodes: Allow use of out of tree cgen source directory

sim/opcodes: Allow use of out of tree cgen source directory

When configuring with '--enbale-cgen-maint' the default for both the
opcodes/ and sim/ directories is to assume that the cgen source is
within the binutils-gdb source tree as binutils-gdb/cgen/.

In the old cvs days, this worked well, as cgen was just another
sub-module of the single cvs repository and could easily be checked
out within the binutils-gdb directory, and managed by cvs in the
normal way.

Now that binutils-gdb is in git, while cgen is still in cvs, placing
the cgen respository within the binutils-gdb tree is more troublesome,
and it would be nice if the two tools could be kept separate.

Luckily there is already some initial code in the configure.ac files
for both opcodes/ and sim/ to support having cgen be located outside
of the binutils-gdb tree, however, this was speculative code written
imagining a future where cgen would be built and installed to some
location.

Right now there is no install support for cgen, and so the configure
code in opcodes/ and sim/ doesn't really do anything useful.  In this
commit I repurpose this code to allow binutils-gdb to be configured so
that it can make use of a cgen source directory that is outside of the
binutils-gdb tree.

With this commit applied it is now possible to configure and build
binutils-gdb like this:

    /path/to/binutils-gdb/src/configure --enable-cgen-maint=/path/to/cgen/src/cgen/
    make all-opcodes
    make -C opcodes run-cgen-all

Just in case anyone is still using cgen inside the binutils-gdb tree,
I have left the default behaviour of '--enable-cgen-maint' (with no
parameter) unchanged, that is it looks for the cgen directory as
'binutils-gdb/cgen/'.

opcodes/ChangeLog:

	* configure.ac (enable-cgen-maint): Support passing path to cgen
	source tree.
	* configure: Regenerate.

sim/ChangeLog:

	* common/acinclude.m4 (enable-cgen-maint): Support passing path to
	cgen source tree.
	* cris/configure: Regenerate.
	* frv/configure: Regenerate.
	* iq2000/configure: Regenerate.
	* lm32/configure: Regenerate.
	* m32r/configure: Regenerate.
	* or1k/configure: Regenerate.
	* sh64/configure: Regenerate.