x86/syscall: Don't force use of indirect calls for system calls

Make <asm/syscall.h> build a switch statement instead, and the compiler can
either decide to generate an indirect jump, or - more likely these days due
to mitigations - just a series of conditional branches.

Yes, the conditional branches also have branch prediction, but the branch
prediction is much more controlled, in that it just causes speculatively
running the wrong system call (harmless), rather than speculatively running
possibly wrong random less controlled code gadgets.

This doesn't mitigate other indirect calls, but the system call indirection
is the first and most easily triggered case.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>