target/ppc: Implement ppc_cpu_record_sigsegv

target/ppc: Implement ppc_cpu_record_sigsegv

Record DAR, DSISR, and exception_index.  That last means
that we must exit to cpu_loop ourselves, instead of letting
exception_index being overwritten.

This is exactly what the user-mode ppc_cpu_tlb_fill does,
so simply rename it as ppc_cpu_record_sigsegv.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>