xtensa: use "m" constraint instead of "a" in uaccess.h assembly

Use "m" constraint instead of "r" for the address, as "m" allows
compiler to access adjacent locations using base + offset, while "r"
requires updating the base register every time.
Use %[mem] * 0 + v to replace offset part of %[mem] expansion with v.
It is impossible to change address alignment through the offset part on
xtensa, so just ignore offset in alignment checks.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>