sim/lm32: fix some missing function declaration warnings

In the lm32 simulator, I was seeing some warnings about missing
function declarations.

The lm32 simulator has a weird header structure, in order to pull in
the full cpu.h header we need to define WANT_CPU_LM32BF.  This is done
in some files, but not in others.  Critically, it's not done in some
files that then use functions declared in cpu.h

In this commit I added the missing #define so that the full cpu.h can
be included.

After doing this there are still a few functions that are used
undeclared, these functions appear to be missing any declarations at
all, so I've added some to cpu.h.

With this done all the warnings when compiling lm32 are resolved for
both gcc and clang, so I've removed the SIM_WERROR_CFLAGS line from
Makefile.in, this allows lm32 to build with -Werror.