riscv.pm: Add improved error messages to read_reg code

riscv.pm: Add improved error messages to read_reg code

On systems where Devel::StackTrace is available, we can use this module
to create more usable error messages. Further, don't print error
messages in case of official register aliases, but simply accept them.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20078)