arch/xtensa: Properly namespace special register API

The Xtensa arch has historically had state/user register accessor
macros with bare three-byte symbol names.  I think this might have
been in the original Cadence-contributed arch integration, but I'm not
sure.  In any case they also exist in the same names in vendor
HAL/toolchain code and are causing collisions.  We never should have
had these symbols exposed in our header.

Put them under an XTENSA_ prefix to decollide.

Signed-off-by: Andy Ross <andyross@google.com>