[pthreads] Allow using CLOCK_BOOTTIME in pthread_condattr_setclock()

Signed-off-by: Hannu Mallat <hannu.mallat@jollamobile.com>

Allow CLOCK_BOOTTIME in pthread_condattr_setclock() so that glib2 can use CLOCK_BOOTTIME as thread timer while using it with g_get_monotonic_time().

Since the id for CLOCK_BOOTTIME is 7, had to resize the bit field holding clock id in the pthread_cond_t structure.