Don't require M_SYSCALL in sandbox.c

In sandbox.c, we have a platform-dependent M_SYSCALL macro that is used to extract the syscall from a ucontext_t pointer.

But we only use this value for debugging. Perhaps instead we should make it optional, so that platforms where we don't have it defined can still build with m_syscall.

See also #32904 (moved) and #30987 (moved)