vdso/math64: Use OPTIMIZER_HIDE_VAR() in __iter_div_u64_rem()

__iter_div_u64_rem() open-codes an empty asm() to keep the compiler from
turning the subtraction loop into a modulo operation.  That is what
OPTIMIZER_HIDE_VAR() is for, so use it and reflow the comment.

The macro lives in linux/compiler.h, which this header did not include.
vdso/math64.h is reached from linux/math64.h, whose includes are
linux/types.h, linux/math.h, asm/div64.h and vdso/math64.h; none of them
guarantees linux/compiler.h.  x86 happens to get it through
asm/div64.h -> linux/log2.h, m68k's asm/div64.h only pulls in
asm-generic/div64.h and linux/types.h, so the macro is undeclared there
and the build fails with -Werror=implicit-function-declaration.

Include linux/compiler.h so the header no longer depends on its includer.
This is a no-op for the vDSO, where lib/vdso/gettimeofday.c already gets
it through linux/build_bug.h, and vdso/unaligned.h sets the same
precedent by including linux/compiler_types.h.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20261002153104.1347978-2-zhanxusheng@xiaomi.com
Closes: https://lore.kernel.org/oe-kbuild-all/202609300626.zwNjKaO6-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202609301300.uAawkHuh-lkp@intel.com/

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