arch: xtensa: rename processor state save and restore calls

_xtos_pso_savearea and _xtos_core_restore_nw have been renamed to
xthal_pso_savearea and xthal_core_restore_nw in recent Xtensa
toolchains. Rename them in reset_vector.S to prevent linker errors
when building Zephyr with Xtensa toolchain.

Signed-off-by: Tahsin Mutlugun <Tahsin.Mutlugun@analog.com>