i#1568 Mac64 TLS: Use pthread TLS slots for DR and clients

[derekbruening]

Uses pthread_key_create() to allocate enough contiguous and aligned TLS
slots to fit our os_local_state_t struct. This makes it easier to share
Linux code for Mac64.

Keeps the scheme from ce8e803 of storing a pointer to the base of
os_local_state_t in TLS slot 6. This is indirection we don't need with the
entire os_local_state_t struct in TLS but it is not clear we can take that
many TLS slots for large applications, so I'm leaving this mixture until
we're sure which direction to go in.

Disables the options -mangle_app_seg and -safe_read_tls_init for Mac64.

Issue: #1568, #1979