Emit: Factor out parts needed for emitting an AstCFunc

Factored out bits from V3EmitC.cpp that is required to emit a whole
(non-trace) AstCFunc. This is mostly what used to be the EmitCStmts
class plus relevant bits from EmitCImp. These now live in EmitCFunc,
which is reusable by anything that needs to emit a regular AstCFunc
(differences in tracing to be addressed later). EmitCImp now extends
EmitCFunc instead of EmitCStmts. No functional change intended.

src/Makefile_obj.in

@@ -182,6 +182,7 @@ RAW_OBJS = \
 	V3EmitC.o \
 	V3EmitCBase.o \
 	V3EmitCConstPool.o \
+	V3EmitCFunc.o \
 	V3EmitCInlines.o \
 	V3EmitCSyms.o \
 	V3EmitCMake.o \