Add IEEE half-precision data type.

After discussion with clang and gcc community, we choose
C ISO/IEC TS 18661-3 `_Float16` rather than redefine
a different behavior type `__fp16` in RISC-V.

1. ARM folks are working on deprecating `__fp16`. [1]
2. In Clang, `_Float16` is available in both C and C++ mode. [2]
3. In GCC, `_Float16` is only avaiable in C mode but GCC folks
think making it available in C++ as GCC extension is doable. [3]

[1] https://lists.llvm.org/pipermail/cfe-dev/2021-March/067850.html
[2] https://gcc.gnu.org/pipermail/gcc/2021-March/234981.html
[3] https://gcc.gnu.org/pipermail/gcc/2021-March/234972.html

riscv-elf.md

@@ -378,6 +378,7 @@ There are two conventions for C type sizes and alignments.
     long long            |  8            |  8
     __int128             | 16            | 16
     void *               |  8            |  8
+    _Float16             |  2            |  2
     float                |  4            |  4
     double               |  8            |  8
     long double          | 16            | 16
@@ -397,6 +398,7 @@ There are two conventions for C type sizes and alignments.
     long                 |  4            |  4
     long long            |  8            |  8
     void *               |  4            |  4
+    _Float16             |  2            |  2
     float                |  4            |  4
     double               |  8            |  8
     long double          | 16            | 16
@@ -420,6 +422,8 @@ arguments are either `0` (`false`) or `1` (`true`).
 
 A null pointer (for all types) has the value zero.
 
+`_Float16` is as defined in the C ISO/IEC TS 18661-3 extension.
+
 `_Complex` types have the same layout as a struct containing two fields of the
 corresponding real type (`float`, `double`, or `long double`), with the first
 member holding the real part and the second member holding the imaginary part.