Expand type support in cfloat.h

- Add support for unsigned floats
- Other improvements to cfloat.h
- Add cfixed template for fixed point types
- Reorganise cmicroscaling with floating point types in cfloat.h and
  fixed point types in cfixed.h
- Add missing specialisations for numeric_limits

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ia471bda84f1784ccf49277f93df2c2e712af5b36