hexagon: Remove unusable symbols from the ptrace.h uapi

Kernel-internal prototypes, references to current_thread_info()
and code hidden behind a CONFIG_HEXAGON_ARCH_VERSION switch are
certainly not usable in userspace, so this should not reside
in a uapi header. Move the code into an internal version of
ptrace.h instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>