tests: arch: arm_thread_swap: Fix FPSCR validation for ARMv8.1-M

This commit fixes the FPSCR register initialisation validation test
for the ARMv8.1-M architecture.

For ARMv8.1-M, the newly added LTPSIZE field in the FPSCR may always
read the value of 4 when the M-Profile Vector Extension (MVE) is not
implemented or FP context is not active, so we must ignore its value
when validating the FPSCR register initialisation.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>