[openmp] Apply a mingw/64 bit codepath to arm64ec too (#209297) When targeting arm64ec, neither KMP_ARCH_X86_64 nor KMP_ARCH_AARCH64 is defined. This fixes a compilation warning when building openmp for mingw/arm64ec. GitOrigin-RevId: 8eb6e260047bd2f69d9d2c1f0967ae59f7ec5374