powerpc: add CFUNC assembly label annotation

This macro is to be used in assembly where C functions are called.
pcrel addressing mode requires branches to functions with a
localentry value of 1 to have either a trailing nop or @notoc.
This macro permits the latter without changing callers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add dummy definitions to fix selftests build]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408021752.862660-5-npiggin@gmail.com