Cleaned up pthread rwlocks implementation.

- used underscore_style_for_vars
- extracted time related functionality into a function
- cleaned up style
- removed unused fields from pthread_rwlock_t on LP64
- changed reservation in pthread_rwlock_t so that the size of the
structure equals glibc version

Bug: 8133149

Change-Id: I84ad3918678dc7f5e6b3db9b7e9b0899d3abe9cd