s390: ptrace: hard-code "s390x" instead of UTS_MACHINE

s390 uses the UTS_MACHINE defined arch/s390/Makefile as follows:

  UTS_MACHINE     := s390x

We do not need to pass the fixed string from the command line.
Hard-code user_regset_view::name, like many other architectures do.

Link: https://lkml.kernel.org/r/20200413013113.8529-1-masahiroy@kernel.org


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>