compiler.h: Move compiletime_assert() macros into compiler_types.h

compiler.h: Move compiletime_assert() macros into compiler_types.h

The kernel test robot reports that moving READ_ONCE() out into its own
header breaks a W=1 build for parisc, which is relying on the definition
of compiletime_assert() being available:

  | In file included from ./arch/parisc/include/generated/asm/rwonce.h:1,
  |                  from ./include/asm-generic/barrier.h:16,
  |                  from ./arch/parisc/include/asm/barrier.h:29,
  |                  from ./arch/parisc/include/asm/atomic.h:11,
  |                  from ./include/linux/atomic.h:7,
  |                  from kernel/locking/percpu-rwsem.c:2:
  | ./arch/parisc/include/asm/atomic.h: In function 'atomic_read':
  | ./include/asm-generic/rwonce.h:36:2: error: implicit declaration of function 'compiletime_assert' [-Werror=implicit-function-declaration]
  |    36 |  compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long), \
  |       |  ^~~~~~~~~~~~~~~~~~
  | ./include/asm-generic/rwonce.h:49:2: note: in expansion of macro 'compiletime_assert_rwonce_type'
  |    49 |  compiletime_assert_rwonce_type(x);    \
  |       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  | ./arch/parisc/include/asm/atomic.h:73:9: note: in expansion of macro 'READ_ONCE'
  |    73 |  return READ_ONCE((v)->counter);
  |       |         ^~~~~~~~~

Move these macros into compiler_types.h, so that they are available to
READ_ONCE() and friends.

Link: http://lists.infradead.org/pipermail/linux-arm-kernel/2020-July/587094.html
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Will Deacon <will@kernel.org>