Drop support for -fcheck-pointer-bounds

Drop support for -fcheck-pointer-bounds

GCC has removed the -fcheck-pointer bounds option, and the Linux
kernel has also removed support for Intel MPX, so there’s no point
to keeping this debugging option within Emacs.
* src/bytecode.c (BYTE_CODE_THREADED):
* src/lisp.h (DEFINE_LISP_SYMBOL, XSYMBOL, make_lisp_symbol):
Assume __CHKP__ is not defined.
* src/ptr-bounds.h: Remove.  All uses of ptr_bounds_clip,
ptr_bounds_copy, ptr_bounds_init, ptr_bounds_set removed.