update compiler_rt README and prettify it

This documents status of routines and adds the next work item
"Decimal float library routines", which are only recommended for
binary data. Complete absence of tests is also documented.
This does not document the various aliases, e.g. those for ARM.

Missing Integer library routines:
- __addvsi3
- __addvdi3
- __addvti3
- __addvdi3
- __addvti3
- __subvsi3
- __subvdi3
- __subvti3
- __subvdi3
- __subvti3
- __mulvsi3
- __mulvdi3
- __mulvti3
- __mulvdi3
- __mulvti3

Missing floating library routines:
- __powisf2
- __powidf2
- __powitf2
- __powixf2

Missing routines for symbol-level compatibility to gcc:
- __ashlsi3
- __ashrsi3
- __lshrsi3

Author: Jan Philipp Hafer