Auto merge of #18031 - jhlin:android-build-on-macos, r=mbrubeck

Fix Android build errors on macOS.

- Add sysroot path to environment variable 'CPPFLAGS':
  When checking C preprocessor, the 'configure' script of libbacktrace uses 'CPPFLAGS' rather than 'CFLAGS' and doesn't get the correct search path. (#15758)
  This check passes on Linux because the '/lib/cpp' fallback is available there.

- Introduce CMake toolchain file for Android cross compiling:
  CMake needs several variables [1] to cross compile for Android.
  It works (accidentally) on Linux because cmake-rs sets compilers correctly and binutils for Linux & Android are pretty much the same.

[1] https://cmake.org/cmake/help/v3.7/manual/cmake-toolchains.7.html#cross-compiling-for-android-with-the-ndk

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15758 (github issue number if applicable).

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