fix: repair the unified emitter's declarations

Compiling the merged sources with the Open Watcom install on this host
(C:\WATCOM19\binnt) found four things the merge got wrong, none of which any
amount of reading would have caught reliably:

- FE_M7_FLOW_CAP and <stdlib.h> lived in check_m7.c's preamble, above the
  textual include, and were dropped with the wrapper.
- emit_error_return takes a const char *, not a FeNode *; the hand-written
  forward declaration disagreed with the definition.
- emit_match was never defined by the M7 half, only called, so renaming it
  alongside the other delegating pairs left a declared-but-undefined static.
- type_needs_drop and emit_lvalue are used a few hundred lines before the
  declaration block, so their declarations had to be hoisted.

Also drop two locals that existed only to be cast to void.

All twelve compiler sources now compile with -za -wx -wcd=202 and produce no
warnings. That is a syntax and type check, not verification -- the DOS build
and the milestone suite remain the gate.

fec/check.err

@@ -0,0 +1 @@
+src\check.c(2795): Error! E1118: ***FATAL*** No such file or directory