arch: arm: swap-helper: simplify several assembly expressions

Some assembly simplifications, to make code common for ARMv6
and ARMv7 architecture.

We can use ldrb, directly for reading the SVC encoding; this
removes the need for ANDing the result with 0xff right below.
We remove an immediate value of 0 from an str instruction, as
it's redundant.

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