[SCons] Platform agnostic default toolchain (GNU).

[Faless]

Create the SCons Environment with an empty PLATFORM variable to force the default tools to use the GNU toolchain.

Platform specific toolchains are then setup in our custom tools.

Fixes #937

[DmitriySalnikov]

With this fix, it finally became possible to build a library on Windows.
Thanks!

C:\Android\SDK\ndk\23.2.8568313/toolchains/llvm/prebuilt/windows-x86_64/bin/clang++ -o gen\src\variant\utility_functions.android.template_release.arm64.o -c -std=c++17 --target=aarch64-linux-android21 -march=armv8-a -fPIC -O3 -Igdextension -Iinclude -Igen\include gen\src\variant\utility_functions.cpp
scons: `bin\libgodot-cpp.android.template_release.arm64.a' is up to date.
scons: done building targets.

[akien-mga]

Thanks!