ppc64: Avoid pt_regs struct definition

ppc64: Avoid pt_regs struct definition

Remove pt_regs indirection and instead reference gp_regs directly, this
makes it portable across musl/glibc

Use PT_* constants defined in asm/ptrace.h

Move the file to ppc64 subdir and leave ppc empty

Fixes
../qemu-6.2.0/linux-user/host/ppc64/../ppc/host-signal.h:16:32: error: incomplete definition of type 'struct pt_regs'
    return uc->uc_mcontext.regs->nip;
           ~~~~~~~~~~~~~~~~~~~~^

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220315015740.847370-1-raj.khem@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>