[build] Add the flags to enable cross-compiling the corelibs

Pass the Swift and CMake flags needed to cross-compile Foundation and so on, with
the first example of Android. Add a new flag, --cross-compile-deps-path, which is
used to search for cross-compiled libraries, like libcurl, that the corelibs
depend on. Also add a new flag, --common-swift-flags, to pass additional Swift
flags to the corelibs.

Author: finagolfin

validation-test/BuildSystem/android_cross_compile.test

@@ -1,4 +1,3 @@
-# REQUIRES: standalone_build
 # REQUIRES: OS=linux-gnu
 
 # RUN: %empty-directory(%t)