kekcall ABI

The recommended entrypoint is the syscall instruction at `getppid+0x0a` in libkernel, although in practice any syscall instruction in libkernel can be used.

Calls:
* rax = 0x000000027 -> real getppid
* rax = 0x100000027, rdi = X -> copy dr0-dr3 and dr6-dr7 to user address X
* rax = 0x200000027, rdi = X -> copy dr0-dr3 and dr6-dr7 from user address X
* rax = 0x300000027, rdi = X -> returns rdmsr(X)
* rax = 0x500000027, rdi = pid, rsi = syscno, rdx = X -> call syscall in foreign process, with args read from user address X
