arch: arm: cortex-m: Change character used to mark immediate operand

Change the character used to indicate immediate operands from '$' to '#'
to resolve an "invalid instruction" error when building with clang.

For arm, binutils allows either '#' or '$' to indicate immediate operands.
clang seems to accept '$' for arm in other instances
(my build accepts 'subs r0, r0, $0x02', for example), but in this case it
produces an error that this is an invalid instruction due to the "$0x02"
operand.

Given clang's inconsistent behavior, I'm guessing this is a bug in clang
somewhere, but:

  1. '#' for immediate operands seems to be more standard for arm in
     general and seems to be what is used throughout the rest of Zephyr's
     arm asm code.
  2. Switching out '$' for '#' shouldn't negatively impact other
     toolchains.

As such, switch out the character used to unblock clang builds until this
can be fixed in clang.

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>