Fix missing error handling of pthread_attr_init/getstacksize

* misc.c (GC_init): Explicitly ignore returned value of
pthread_mutexattr_destroy.
* os_dep.c (GC_get_main_stack_base, GC_get_stack_base, GC_dirty_init):
Likewise.
* pthread_support.c (start_mark_threads, pthread_create): Likewise.
* tests/test.c (main): Likewise.
* win32_threads.c (start_mark_threads): Likewise.
* pthread_support.c (pthread_create): ABORT (with the appropriate
message) in case of pthread_attr_getstacksize or pthread_attr_init
failure.
* tests/test.c (main): Print error code and FAIL if pthread_attr_init
or pthread_attr_setstacksize failed (only if GC_PTHREADS).
Fix missing msvc_dbg.h in dist_noinst_HEADERS (Automake)

* include/include.am (dist_noinst_HEADERS): Add msvc_dbg.h entry.
Fix missing cord_pos.h, ec.h among installed headers (Automake)

* cord/cord.am (pkginclude_HEADERS): Add cord_pos.h, ec.h entries.
Add libatmoic_ops as a submodule.

.gitignore:
    Remove libatomic_ops from ignore list.

.gitmodules:
libatomic_ops:
    Add new submodule.