syscalls/core, syscalls/x86: Clean up compat syscall stub naming convention

Tidy the naming convention for compat syscall subs. Hints which describe
the purpose of the stub go in front and receive a double underscore to
denote that they are generated on-the-fly by the COMPAT_SYSCALL_DEFINEx()
macro.

For the generic case, this means:

t            kernel_waitid	# common C function (see kernel/exit.c)

    __do_compat_sys_waitid	# inlined helper doing the actual work
				# (takes original parameters as declared)

T   __se_compat_sys_waitid	# sign-extending C function calling inlined
				# helper (takes parameters of type long,
				# casts them to unsigned long and then to
				# the declared type)

T        compat_sys_waitid      # alias to __se_compat_sys_waitid()
				# (taking parameters as declared), to
				# be included in syscall table

For x86, the naming is as follows:

t            kernel_waitid	# common C function (see kernel/exit.c)

    __do_compat_sys_waitid	# inlined helper doing the actual work
				# (takes original parameters as declared)

t   __se_compat_sys_waitid      # sign-extending C function calling inlined
				# helper (takes parameters of type long,
				# casts them to unsigned long and then to
				# the declared type)

T __ia32_compat_sys_waitid	# IA32_EMULATION 32-bit-ptregs -> C stub,
				# calls __se_compat_sys_waitid(); to be
				# included in syscall table

T  __x32_compat_sys_waitid	# x32 64-bit-ptregs -> C stub, calls
				# __se_compat_sys_waitid(); to be included
				# in syscall table

If only one of IA32_EMULATION and x32 is enabled, __se_compat_sys_waitid()
may be inlined into the stub __{ia32,x32}_compat_sys_waitid().

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180409105145.5364-3-linux@dominikbrodowski.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>