arch: arm: protect r0 in z_arch_switch_to_main_thread() inline ASM

Adding r0 to the clobber list in the inline ASM block of
z_arch_switch_to_main_thread(). This instructs assembler
to not use r0 to store ASM expression operands, e.g. in
the subsequent instruction, msr PSR %1.

We also do a minor optimization with the clearing of R1
before jumping to main.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>