Add new macro for RV64E

`__riscv_64e` is defined analogously to `__riscv_32e`.

Changing the meaning of `__riscv_32e` should not cause any problems
since if we are just testing for existence of the `E` extension,
there is `__riscv_e` preprocessor macro.

Author: a4lg

riscv-c-api.md

@@ -38,7 +38,8 @@ https://creativecommons.org/licenses/by/4.0/.
 | __riscv             | 1     | Always defined.               |
 | __riscv_xlen        | <ul><li>32 for rv32</li><li>64 for rv64</li><li>128 for rv128</ul> | Always defined.             |
 | __riscv_flen        | <ul><li>32 if the F extension is available **or**</li><li>64 if `D` extension available **or**</li><li>128 if `Q` extension available</li></ul> | `F` extension is available. |
-| __riscv_32e         | 1     | `E` extension is available.   |
+| __riscv_32e         | 1     | RV32E is available.           |
+| __riscv_64e         | 1     | RV64E is available.           |
 | __riscv_vector      | 1     | Implies that any of the vector extensions (`v` or `zve*`) is available |
 | __riscv_v_min_vlen    | <N> (see [__riscv_v_min_vlen](#__riscv_v_min_vlen)) | The `V` extension or one of the `Zve*` extensions is available. |
 | __riscv_v_elen     | <N> (see [__riscv_v_elen](#__riscv_v_elen)) | The `V` extension or one of the `Zve*` extensions is available. |