TLS Fastcall WIP

test with
asm volatile ("syscall" : : "D"(0xf0f0000000000001),
"S"(0x0123456789ab));  (should work)

asm volatile ("syscall" : : "D"(0xf0f0000000000001),
"S"(0xdeaddeadbeef));  (should GP fault)