x86: bitops: fix build regression

x86: bitops: fix build regression

This is easily reproducible via CC=clang + CONFIG_STAGING=y +
CONFIG_VT6656=m.

It turns out that if your config tickles __builtin_constant_p via
differences in choices to inline or not, these statements produce
invalid assembly:

    $ cat foo.c
    long a(long b, long c) {
      asm("orb	%1, %0" : "+q"(c): "r"(b));
      return c;
    }
    $ gcc foo.c
    foo.c: Assembler messages:
    foo.c:2: Error: `%rax' not allowed with `orb'

Use the `%b` "x86 Operand Modifier" to instead force register allocation
to select a lower-8-bit GPR operand.

The "q" constraint only has meaning on -m32 otherwise is treated as
"r".  Not all GPRs have low-8-bit aliases for -m32.

Fixes: 18c22b95fce2f ("x86: Fix bitops.h warning with a moved cast")
Reported-by: kernelci.org bot <bot@kernelci.org>
Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Suggested-by: Brian Gerst <brgerst@gmail.com>
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Suggested-by: Ilie Halip <ilie.halip@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>	[build, clang-11]
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-By: Brian Gerst <brgerst@gmail.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Marco Elver <elver@google.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20200508183230.229464-1-ndesaulniers@google.com
Link: https://github.com/ClangBuiltLinux/linux/issues/961
Link: https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
Link: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#x86Operandmodifiers
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>