powerpc/fprobe: fix updated fprobe for function-graph tracer

Since commit 4346ba16 ("fprobe: Rewrite fprobe on function-graph
tracer"), FPROBE depends on HAVE_FUNCTION_GRAPH_FREGS. With previous
patch adding HAVE_FUNCTION_GRAPH_FREGS for powerpc, FPROBE can be
enabled on powerpc. But with the commit b5fa903b ("fprobe: Add
fprobe_header encoding feature"), asm/fprobe.h header is needed to
define arch dependent encode/decode macros. The fprobe header MSB
pattern on powerpc is not 0xf. So, define FPROBE_HEADER_MSB_PATTERN
expected on powerpc.

Also, commit 762abbc0 ("fprobe: Use ftrace_regs in fprobe exit
handler") introduced HAVE_FTRACE_REGS_HAVING_PT_REGS for archs that
have pt_regs in ftrace_regs. Advertise that on powerpc to reuse
common definitions like ftrace_partial_regs().

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Aditya Bodkhe <aditya.b1@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250916044035.29033-2-adityab1@linux.ibm.com