x86_64/entry: Mark all syscalls __noendbr

Since system call dispatch is strictly through direct calls, there is no reason
for the syscalls to be called indirectly. Mark them all __noendbr.

Notably, the sys_call_table[] is purely a lookup for tracing and the like,
nobody actually (should) dispatch anything through it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>