cmake: Deprecate the 2 symbols _SYSCALL_{LIMIT,BAD}

There exist two symbols that became equivalent when PR #9383 was
merged; _SYSCALL_LIMIT and K_SYSCALL_LIMIT. This patch deprecates the
redundant _SYSCALL_LIMIT symbol.

_SYSCALL_LIMIT was initally introduced because before PR #9383 was
merged K_SYSCALL_LIMIT was an enum, which couldn't be included into
assembly files. PR #9383 converted it into a define, which can be
included into assembly files, making _SYSCALL_LIMIT redundant.

Likewise for _SYSCALL_BAD.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>