selftests: sigaltstack: fix -Wuninitialized

Building sigaltstack with clang via:
$ ARCH=x86 make LLVM=1 -C tools/testing/selftests/sigaltstack/

produces the following warning:
warning: variable 'sp' is uninitialized when used here [-Wuninitialized]
if (sp < (unsigned long)sstack ||
^~

Clang expects these to be declared at global scope; we've fixed this in
the kernel proper by using the macro `current_stack_pointer`. This is
defined in different headers for different target architectures, so just
create a new header that defines the arch-specific register names for
the stack pointer register, and define it for more targets (at least the
ones that support current_stack_pointer/ARCH_HAS_CURRENT_STACK_POINTER).

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Link: https://lore.kernel.org/lkml/CA+G9fYsi3OOu7yCsMutpzKDnBMAzJBCPimBp86LhGBa0eCnEpA@mail.gmail.com/
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>