powerpc/lib/sstep: fix 'ptesync' build error

Building tinyconfig with gcc (Debian 11.2.0-16) and assembler (Debian
2.37.90.20220207) the following build error shows up:

{standard input}: Assembler messages:
{standard input}:2088: Error: unrecognized opcode: `ptesync'
make[3]: *** [/builds/linux/scripts/Makefile.build:287: arch/powerpc/lib/sstep.o] Error 1

Add the 'ifdef CONFIG_PPC64' around the 'ptesync' in function
'emulate_update_regs()' to like it is in 'analyse_instr()'. Since it looks like
it got dropped inadvertently by commit 3cdfcbfd ("powerpc: Change
analyse_instr so it doesn't modify *regs").

Cc: stable@vger.kernel.org # v4.14+
Fixes: 3cdfcbfd ("powerpc: Change analyse_instr so it doesn't modify *regs")
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>