Function attribute for standard vector calling convention variant: riscv_vector_cc

Standard vector calling convention variant will only enabled when function has
vector argument or returing value by default, however user may also want to
invoke function without that during a vectorized loop at some
situation, but it will cause a huge performance penalty due to vector
register store/restore.

So user can declare function with this riscv_vector_cc attribute like below,
that could enforce function will use standard vector calling convention
variant.

```c
void foo() __attribute__((riscv_vector_cc));
```

Author: kito-cheng

riscv-c-api.md

@@ -318,6 +318,12 @@ __attribute__((target("arch=+v"))) int foo(void) { return 0; }
 __attribute__((target("arch=+zbb"))) int foo(void) { return 1; }
 ```
 
+### `__attribute__((riscv_vector_cc))`
+
+Functions declared with this attribute will use to the standard vector calling
+convention variant as defined in the RISC-V psABI, even if the function has
+vector arguments or a return value.
+
 ## Intrinsic Functions
 
 Intrinsic functions (or intrinsics or built-ins) are expanded into instruction sequences by compilers.