Eliminate most of collisions in GC_threads on Linux/x64

Eliminate most of collisions in GC_threads on Linux/x64

On some targets (e.g., Linux/x64) pthread_t values always have zero
in the lowest byte (e.g. if pthread_t is actually a page-aligned
pointer) thus causing all the created thread descriptors to have
a collision at GC_threads[0].  This commit mixes 3 lowest bytes
of thread id with a XOR operation (thus greatly reduces the
probability of collisions in GC_threads).

* include/private/pthread_support.h (THREAD_TABLE_SZ): Do not
define if already defined; refine comment.
* pthread_support.c (THREAD_TABLE_INDEX): New macro (similar to that
in win32_threads.c).
* pthread_support.c (GC_new_thread, GC_delete_thread,
GC_delete_gc_thread, GC_lookup_thread): Replace
NUMERIC_THREAD_ID(id)%THREAD_TABLE_SZ to THREAD_TABLE_INDEX(id).
* pthread_support.c (GC_lookup_thread): Remove hv local variable.
* win32_threads.c (THREAD_TABLE_INDEX): Replace (id>>2) to
((id>>16)^(id>>8)^id).
* win32_threads.c [GC_PTHREADS] (GC_lookup_pthread): Define win32_id
local variable to pass it to THREAD_TABLE_INDEX() instead of passing
GET_PTHREAD_MAP_CACHE(id) directly.