The linker detection essentitally invokes `$CC -Wl,--version` to
determine which linker will be used. This invocation works OK for GCC
from the Android NDK, which can find the linker in the same directory;
it doesn't work so well for clang from the NDK, which needs additional
command-line options to point at the directory containing the linker.
We already have extra_toolchain_flags for communicating such options to
other compiler invocations for the target; we should be using
extra_toolchain_flags here as well.
397f70843c