Change riscv_return_value to use RETURN_VALUE_ABI_PRESERVES_ADDRESS

Internally, AdaCore has a test that is equivalent to (really a direct
translation of) gdb.base/gnu_vector.exp.  On 32-bit RISC-V, the
"return" part of this test fails.

Joel tracked this down to riscv_return_value returning
RETURN_VALUE_ABI_RETURNS_ADDRESS.  Using
RETURN_VALUE_ABI_PRESERVES_ADDRESS is more correct here, and fixes the
bug.

I tested this for both 32- and 64-bit RISC-V using the AdaCore
internal test suite, and Andrew Burgess tested it using
gnu_vector.exp.