Fix WOW64 registry mode on Windows 2000 (#10759)

CMake fails to find any registry paths on Windows 2000: according to regmon
it fails with an access denied error. I double checked all the access rights
and they are fine. After checking the access modes on MSDN I found that it
says KEY_WOW64_32KEY / KEY_WOW64_64KEY are not supported on Windows 2000.
CMake does not check if the current system supports Wow64 before applying
these flags.

This commit adds a check for IsWow64Process in kernel32.dll before adding
these flags.

Author: Axel Gembe <ago@bastart.eu.org>
Signed-off-by: Axel Gembe <ago@bastart.eu.org>