Bug 1820947 - Use "new style" sysroot on Android. r=firefox-build-system-reviewers,andi

For a few NDK releases now, the situation has been simplified wrt
headers and libraries, and while we're currently still using things here
and there because we never changed our ways, we can simplify things a
lot by using the new simplified things. This involves:
- Using a --target that contains the Android version, making clang set
  __ANDROID_API__ itself, and makes it look in $sysroot/usr/lib/$target/$ver
  when linking.
- Using the sysroot that is under toolchains/llvm/prebuilt/*.
- Removing the hacks around libstdc++/libc++.

This ends up emptying stlport compiler flags, which allows to remove a
bunch of things.

Differential Revision: https://phabricator.services.mozilla.com/D172039

python/mozbuild/mozbuild/test/configure/test_compile_checks.py

@@ -56,10 +56,6 @@ class BaseCompileChecks(unittest.TestCase):
             def extra_toolchain_flags():
                 return []
 
-            @depends(when=True)
-            def stlport_cppflags():
-                return []
-
             @depends(when=True)
             def linker_ldflags():
                 return []