x86/entry: Remove ABI prefixes from functions in syscall tables

Move the ABI prefixes to the __SYSCALL_[abi]() macros.  This allows removal
of the need to strip the prefix for UML.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200313195144.164260-13-brgerst@gmail.com