[crypto] Add memory output constraints for big-integer inline assembly

[crypto] Add memory output constraints for big-integer inline assembly

The ARM versions of the big-integer inline assembly functions include
constraints to indicate that the output value is modified by the
assembly code.  These constraints are not present in the equivalent
code for the x86 versions.

As of GCC 11, this results in the compiler reporting that the output
values may be uninitialized.

Fix by including the relevant memory output constraints.

Reported-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>