posix: in x86_64 longjmp, preserve rdi/rsi when revoking signal handler context
x64: don't switch to the kernel address space in X64VirtualAddressSpace destructor.

Process::~Process destroys its virtual address space, but this can happen
in the middle of a syscall (eg, waitpid) or at other times where it doesn't
make sense to get an address space switch as a side effect.