[builtins][Android] Fix 386->686 adjustment

At this point in this code:
 - COMPILER_RT_DEFAULT_TARGET_TRIPLE is "i686-linux-android"
 - arch is "i386"

The get_compiler_rt_target function currently turns that into:

   i686-android-linux-android

The ${COMPILER_RT_OS_SUFFIX} is "-android" and redundant, so stop
adding it.

The get_compiler_rt_target() function is used for the
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR mode that isn't normally used with
Android.

Differential Revision: https://reviews.llvm.org/D82150