powerpc: Don't add __powerpc_ prefix to syscall entry points

When using syscall wrappers the __SYSCALL_DEFINEx() and related macros
add a "__powerpc_" prefix to all syscall entry points.

So for example sys_mmap becomes __powerpc_sys_mmap.

This risks breaking workflows and tools that expect the old naming
scheme. At a minimum setting a breakpoint on eg. sys_mmap with gdb no
longer works.

There seems to be no compelling reason to add the "__powerpc_" prefix,
other than that it follows what some other arches do (x86, arm64, s390).

But unlike other arches powerpc doesn't always enable syscall wrappers,
so the syscall entry points can change name depending on CONFIG options.

For those reasons drop the "__powerpc_" prefix, reverting to the
existing naming.

Doing so reveals two prototypes in signal.h that have the incorrect type
when syscall wrappers are enabled. There are already prototypes for both
functions in syscalls.h, so drop the ones from signal.h.

Fixes: 7e92e01b

 ("powerpc: Provide syscall wrapper")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221006135940.1223988-1-mpe@ellerman.id.au