Move typically true check for static syscall out of syscall_thread_enter/exit functions into an inline.

[mjg]

Summary

Move typically true check for static syscall out of syscall_thread_enter/exit functions into an inline.

Prefix original functions with an _.

According to ministat this gives roughly a 4% speedup when calling getuid()
in a loop (syscall_timing benchmark found in tools/tools).

I was considering getting rid of 'td' argument as well as it is unused.

[mjg]

the idea is half-crap - this code still remains in the hot path, even though it can be eliminated. this can be accomplished by providing a extra system call table and a proxy handler installed in the regular one. the proxy handler will inspect regs and whatnot and will get the syscall number + args from there. it can then proceed to do the crappery to safely call the syscall.