Remove arch specific _stp_probing_32bit_app() and _stp_get_regparm().

Remove arch specific _stp_probing_32bit_app() and _stp_get_regparm().

Both functions were doing more than necessary and didn't actually need
access to the current regs. _STP_PROBE_STATE_USER_MODE tells us whether
the current probe triggered in user mode. _stp_is_compat_task() tells
us whether the current task is a 32-on-64 process.

* runtime/compatdefs.h (_stp_is_compat_task): Defined also when CONFIG_COMPAT
  is undefined (to always return false).
* runtime/loc2c-runtime.h: u_fetch_register() and u_store_register() are
  only called from user space dwarf context. Only needs _stp_is_compat_task()
  test.
* runtime/regs.c: Remove arch specific _stp_probing_32bit_app() and
  _stp_get_regparm() functions.
* tapset/i386/registers.stp (_stp_arg): Check regparm directly.
* tapset/powerpc/registers.stp (probing_32bit_app): Use _stp_is_compat_task()
  test.
* tapset/s390/registers.stp (probing_32bit_app): Likewise.
* tapset/x86_64/registers.stp (_stp_arg): Check regparm directly.
  (probing_32bit_app): Use _stp_is_compat_task() test.
  (regparm): Likewise.