Remove API symbols renaming in WCC_MAKEFILE

Remove API symbols renaming in WCC_MAKEFILE

GC_is_marked, GC_incr_bytes_allocd, GC_incr_bytes_freed are now
declared as GC_API, the so symbols renaming by the linker is not needed.

GC_generic_malloc_words_small does not exist anymore.

* WCC_MAKEFILE [MAKE_AS_DLL] (gc.dll): Remove "append $*.lnk export"
for GC_is_marked, GC_incr_bytes_allocd, GC_incr_bytes_freed,
GC_generic_malloc_words_small.
* WCC_MAKEFILE (gc.lib): Adjust indentation.
* WCC_MAKEFILE [MAKE_AS_DLL] (gctest.exe): Remove "append $*.lnk import"
for GC_is_marked.
* WCC_MAKEFILE [MAKE_AS_DLL] (test_cpp.exe): Remove
"append $*.lnk import" for GC_incr_bytes_allocd, GC_incr_bytes_freed,
GC_generic_malloc_words_small.