crypto/arm_arch.h: add a variable declaration

Add this variable declaration to prevent
"-Werror,-Wmissing-variable-declarations" error from compiler.
This error currently only happens on clang.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15240)