RISC-V: Add tests for constraints "i" and "s"

RISC-V: Add tests for constraints "i" and "s"

The constraints "i" and "s" can be used with a symbol that binds
externally, e.g.
```
namespace ns { extern int var, a[4]; }
void foo() {
  asm(".pushsection .xxx,\"aw\"; .dc.a %0; .popsection" :: "s"(&ns::var));
  asm(".reloc ., BFD_RELOC_NONE, %0" :: "s"(&ns::a[3]));
}
```

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/asm-raw-symbol.c: New test.