Add definition of enum storage type

Some APIs use enum types in their definitions. The C specification does not
define the size of the integer type used for storing and passing enum type
variables.

Provide a definition matching GCC's default behavior.

Signed-off-by: Heinrich Schuchardt <[email protected]>

Author: xypron

riscv-elf.md

@@ -447,6 +447,10 @@ A null pointer (for all types) has the value zero.
 
 `_Float16` is as defined in the C ISO/IEC TS 18661-3 extension.
 
+`enum` types use int as storage type if the value range fits into the value
+range of signed int or into the value range of unsigned int. If the value
+range is larger, they use long long as storage type.
+
 `_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.