Do not use pthread_getattr_np if NO_PTHREAD_GETATTR_NP specified

* include/private/gcconfig.h (NO_PTHREAD_GETATTR_NP): Define new macro
for NaCl.
* os_dep.c (GC_get_main_stack_base, GC_get_stack_base): Test
NO_PTHREAD_GETATTR_NP instead of NACL (i.e., do not use
pthread_getattr_np if NO_PTHREAD_GETATTR_NP).
Do not include sigcontext.h if NO_SIGCONTEXT_H (Linux)

* os_dep.c: Do not include [asm/]sigcontext.h (and linux/version.h)
if NO_SIGCONTEXT_H is defined (only if LINUX and not POWERPC).