lkdtm/bugs: Do not confuse the clang/objtool with busy wait loop

lkdtm/bugs: Do not confuse the clang/objtool with busy wait loop

Since commit eb972eab0794 ("lkdtm/bugs: Add cases for BUG and PANIC
occurring in hardirq context"), building with clang for x86_64 results
in the following warnings:

vmlinux.o: warning: objtool: lkdtm_PANIC_IN_HARDIRQ(): unexpected end of section .text.lkdtm_PANIC_IN_HARDIRQ
vmlinux.o: warning: objtool: lkdtm_BUG_IN_HARDIRQ(): unexpected end of section .text.lkdtm_BUG_IN_HARDIRQ

caused by busy "while (wait_for_...);" loops. Add READ_ONCE() and
cpu_relax() to better indicate the intention and avoid any unwanted
compiler optimisations.

Fixes: eb972eab0794 ("lkdtm/bugs: Add cases for BUG and PANIC occurring in hardirq context")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512190111.jxFSqxUH-lkp@intel.com/
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>