Remove stubborn.c file

Remove stubborn.c file
(code refactoring)

* BCC_MAKEFILE (XXXOBJS): Remove XXXstubborn.obj entry.
* CMakeLists.txt (SRC): Remove stubborn.c entry.
* Makefile.am (libgc_la_SOURCES): Likewise.
* Makefile.direct (CSRCS): Likewise.
* PCR-Makefile (CSRC): Likewise.
* build/s60v3/libgc.mmp (SOURCE): Likewise.
* doc/README.Mac (Files to build the GC libraries): Likewise.
* Makefile.direct (OBJS): Remove stubborn.o entry.
* PCR-Makefile (COBJ): Likewise.
* SMakefile.amiga (OBJS): Likewise.
* digimars.mak (OBJS): Likewise.
* NT_MAKEFILE (OBJS): Remove stubborn.obj entry.
* OS2_MAKEFILE (OBJS): Likewise.
* WCC_MAKEFILE (OBJS): Likewise.
* SMakefile.amiga (stubborn.o): Remove rule.
* digimars.mak (stubborn.obj): Likewise.
* extra/gc.c: Do not include stubborn.c.
* gc.mak: Remove rules for stubborn.obj and stubborn.sbr.
* mallocx.c [MANUAL_VDB] (GC_dirty): Move the function declaration from
stubborn.c.
* mallocx.c (GC_malloc_stubborn, GC_change_stubborn,
GC_end_stubborn_change): Move the function implementation from
stubborn.c.
* stubborn.c: Delete file.