Now that CMAKE_HOST_SYSTEM_NAME and CMAKE_SYSTEM_NAME are set by default to
Android in the Termux app, make the needed tweaks. Some tests were adapted
to work natively on Android too, adds sys/cdefs.h to the Bionic modulemap,
and includes the start of native Android platform support in the build-script.
...by crawling it and looking for module interfaces. This is the tool
Apple is using for the "prebuilt-modules" directory in the toolchain
in Xcode.app, which avoids even a first-use cost for importing modules
from Apple SDKs as long as the SDKs haven't changed. We've been
landing changes to support this model on the compiler side, so the
tool to generate the "prebuilt" compiled modules belongs in the
compiler repo too. (In theory this could be used with open-source
toolchains as well, or for alternate SDKs.)
