powerpc/ftrace: Use $(CC_FLAGS_FTRACE) when disabling ftrace

Rather than open-coding -pg whereever we want to disable ftrace, use the
existing $(CC_FLAGS_FTRACE) variable.

This has the advantage that it will work in future when we use a
different set of flags to enable ftrace.

Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>