Clarify address of struct return value is not required to be returned

The psABI didn't specify whether the address should be returned or not.
GCC and LLVM are implemented differently here; GCC will return the
address in a0, but LLVM won't due to it not being specified.

It's a potential ABI incompatible change for GCC, but fortunately GCC
doesn't rely on that behavior and will rematerialise the address rather
than using the value of a0.

The following code can demonstrate this issue:

```
struct int4 {
   int   values[4];
};

struct int4 i4a;

struct int4 foo (struct int4 arg1)
{
  asm volatile ("li a0, 100 #force clobber a0":::"a0");
  return arg1;
}
```

Fixes #238

Co-authored-by: Jessica Clarke <[email protected]>

Author: kito-cheng

riscv-cc.adoc

@@ -155,6 +155,10 @@ type would be passed.  If such an argument would have been passed by
 reference, the caller allocates memory for the return value, and passes the
 address as an implicit first parameter.
 
+NOTE: There is no requirement that the address be returned from the function
+and so software should not assume that a0 will hold the address of the return
+value on return.
+
 The stack grows downwards (towards lower addresses) and the stack pointer shall
 be aligned to a 128-bit boundary upon procedure entry.
 The first argument passed on the stack is located at offset zero of the stack pointer