Implement a float16 directive for assembling 16 bit IEEE 754 floating point numbers...

Implement a float16 directive for assembling 16 bit IEEE 754 floating point numbers for the AArch64 assembler.

The syntax of the directive is:

     .float16 <0-n decimal numbers>
e.g.
     .float16 0.5
     .float16 10.2, NaN, 452.09

The floats will always be encoded using the binary16 format as described in the
IEEE 754-2008 standard. There is no need to support Arm's alternative half-precision
format since AArch64 only supports the IEEE format.

gas	* config/tc-aarch64.c: Add float16 directive and add "Hh" to
	acceptable float characters.
	* doc/c-aarch64.texi: Documentation for float16 directive.
	* testsuite/gas/aarch64/float16-be.d: New test.
	* testsuite/gas/aarch64/float16-le.d: New test.
	* testsuite/gas/aarch64/float16.s: New test.
	* NEWS: Add NEWS entry.