lib/os/heap: Compiler bug workaround on ARC

This is a workaround for a compiler bug on (at least) GCC 12.1.0 in
Zephyr SDK 0.15.1.  The optimizer generates this function with a last
instruction that is an unconditional branch (a tail call into the
chunk_set() handling).  But that means that the NEXT instruction gets
decoded as part of the branch delay slot, but that instruction isn't
part of this function!  Some instructions aren't legal in branch delay
slots.  One of those is ENTER_S, which is a very common entry
instruction for whatever function the linker places after us.  It
seems like the compiler doesn't understand this problem.  Stuff a NOP
in to guarantee the code is legal.

Comment above is duplicated in the code.  The workaround is
straightforward once the issue is understood, but the path to get here
was hilariously weird.

Fixes #54720

Signed-off-by: Andy Ross <andyross@google.com>