add a callback to dump+verify lowering steps

This allows the fronted to register a dump+verify callback for the
lowering transformations performed by the backends.
lower_calls: cleanup, fix some CopyB removal edge cases

When killing the CopyB we have to use the CopyBs memory input instead of
the Call memory input. To do this I had to rework/improve a previous hack/fix
for const calls with compound return.
correctly abort confirm insertion in complicated cases

This fixes a crash bug in C/cp_error077.c
statev documemtation clarification
Fixed some warning for optimize build.
ia32: remove support for negated symconsts

Assembler/linker never supported this anyway.
cleanup, use C99