Workaround '#error' cppcheck messages in backgraph and private headers

Workaround '#error' cppcheck messages in backgraph and private headers

* backgraph.c [!DBG_HDRS_ALL || ALIGNMENT!=CPP_WORDSZ/8]: Skip #error
directive if CPPCHECK.
* include/private/dbg_mlc.h [(KEEP_BACK_PTRS || MAKE_BACK_GRAPH)
&& SHORT_DBG_HDRS]: Likewise.
* include/private/gc_atomic_ops.h [!GC_BUILTIN_ATOMIC && (!AO_HAVE_load
|| !AO_HAVE_store)]: Likewise.
* include/private/gc_hdrs.h [CPP_WORDSZ!=32 && CPP_WORDSZ<36]: Likewise.
* include/private/gc_priv.h [HBLKSIZE]: Likewise.
* include/private/gcconfig.h [SPARC && LINUX && !__ELF__]: Likewise.
* include/private/gcconfig.h [I386 && (__LP64__ || _WIN64)]: Likewise.
* include/private/gcconfig.h [IA64 && HPUX && !_ILP32 && !_LP64]:
Likewise.
* include/private/gcconfig.h [CPP_WORDSZ!=32 && CPP_WORDSZ!=64]:
Likewise.