cl_ext_float_atomics (#552)

* initial version of the cl_ext_float_atomics extension

* add asciidoctor attribute for section anchors

* added issues for special floating-point behavior

* minor formatting fix for line wrapping

* documented interactions with compiler options affecting floating-point behavior

* resolved all open issues
updated extension template with ext extension conventions

* final draft, assign enums and update the XML file

* editorial updates

* a few more editorial improvements

* fix typo: _mas -> _max

* clarify support for the generic address space built-ins

The generic address space built-ins will require support for both
the global and local address space built-ins, not support for only
one of the global or local address space.  See discussion on issue.

* update contributors

* correct SPIR-V extension dependencies for float atomic add

* fix usage of a few missing asciidoctor attributes

* clearly document SPIR-V extension dependencies

SPV_EXT_shader_atomic_float16_add requires SPV_EXT_shader_atomic_float_add, so if an implementation supports fp16 atomic adds then it must accept SPIR-V modules that declare both extensions.

* clarify that there is no mandated minimum float atomic capability

Author: bashbaug