Code refactoring of Emscripten platform support (single-threaded)

Code refactoring of Emscripten platform support (single-threaded)

* alloc.c (min_bytes_allocd): Test STACK_NOT_SCANNED macro instead
of __EMSCRIPTEN__ (stack size to scan is zero if STACK_NOT_SCANNED).
* include/private/gcconfig.h (ALIGNMENT): Remove duplicate definition
for _EMSCRIPTEN__.
* include/private/gcconfig.h (STACK_NOT_SCANNED): New macro defined
for __EMSCRIPTEN__ target (in addition to  OS_TYPE, CPP_WORDSZ,
ALIGNMENT, DATASTART, DATAEND).
* mach_dep.c (GC_push_regs): Test STACK_NOT_SCANNED macro instead
of __EMSCRIPTEN__ (push nothing if STACK_NOT_SCANNED).
* mark_rts.c (GC_push_roots): Test STACK_NOT_SCANNED macro instead
of __EMSCRIPTEN__ (do not call GC_push_regs_and_stack if
STACK_NOT_SCANNED); mark cold_gc_frame argument as potentially unused.
* misc.c (GC_clear_stack): Test STACK_NOT_SCANNED macro instead
of __EMSCRIPTEN__ (do not clear stack if STACK_NOT_SCANNED).
* misc.c (GC_clear_stack): Reformat code.