Minimal support for ISO/IEC TS 18661-3.

Minimal support for ISO/IEC TS 18661-3.

 - GCC will set __FLT_EVAL_METHOD__ to 16 if __fp16 supported, e.g.
   cortex-a55/aarch64.
   - $ aarch64-unknown-elf-gcc -v 2>&1 |grep version
     gcc version 9.2.0 (GCC)
   - $ aarch64-unknown-elf-gcc  -E -dM -mcpu=cortex-a55 - < /dev/null  |grep FLT_EVAL_METHOD
     #define __FLT_EVAL_METHOD__ 16
     #define __FLT_EVAL_METHOD_TS_18661_3__ 16
     #define __FLT_EVAL_METHOD_C99__ 16
 - The behavior of __FLT_EVAL_METHOD__ == 16 is same as
   __FLT_EVAL_METHOD__ == 0 except for float16_t, but newlib didn't
   support float16_t.

ISO/IEC TS 18661-3:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2405.pdf

V2 Changes:
- List Howland, Craig D as co-author since he provide the draft of comment
  in math.h.

Co-authored-by: "Howland, Craig D" <howland@LGSInnovations.com>