[vcpkg scripts] Restore original intention of CMAKE_CROSSCOMPILING

[WangWeiLin-MV]

Motivation

When cross compiling, setting CMAKE_CROSSCOMPILING to empty leads confusion.

• The host dependency will still build
• The port script executes cross-compile behavior since VCPKG_CROSSCOMPILING still 1(TRUE)
• The toolchain script executes native-compile behavior with cross-compiler

So, no longer reset CMAKE_CROSSCOMPILING when cross compiling.

Impact

When the option --host-triplet is set same with target, CMAKE_CROSSCOMPILING is TRUE while VCPKG_CROSSCOMPILING is FALSE.

Previous discussion

See https://github.com/microsoft/vcpkg/issues/40474#issuecomment-2293320077

~CMake STREQUAL is case-sensitive, change ARM64 to arm64~ ~https://cmake.org/cmake/help/latest/variable/CMAKE_APPLE_SILICON_PROCESSOR.html~