[cmake] Fix native tooling when cross-compiling on Linux

At least as of CMake 3.20.3, the CMake platform file for Linux doesn't
define the file type prefix and suffix variables, relying on them being
implicitly empty when they're unset. If we're cross-compiling targeting
Windows on a Linux machine, the values of these prefixes and suffixes
populated by the Windows platform file will still be set after including
the Linux platform file, so we'll incorrectly assume the ".exe" suffix
for the host machine. Explicitly unset the variables before including
the platform file, to prevent any previous values from leaking. Thanks
@beanz for suggesting the fix.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D108473