Configurations/15-android.conf: detect NDK llvm-ar.

This excluses user from additional PATH adjustments in case NDK has
llvm-ar.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7443)

(cherry picked from commit ddf2e8c8)