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

This excluses user from additional PATH adjustments in case NDK has
llvm-ar.
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7443)

(cherry picked from commit ddf2e8c8ccad49cff0d6a0a0b26ca892ec4ce33a)

Configurations/15-android.conf

@@ -94,6 +94,11 @@
                         .  "/$tritools-4.9/prebuilt/$host";
                 $user{CC} = "clang" if ($user{CC} !~ m|clang|);
                 $user{CROSS_COMPILE} = undef;
+                if (which("llvm-ar") =~ m|^$ndk/.*/prebuilt/([^/]+)/|) {
+                    $user{AR} = "llvm-ar";
+                    $user{ARFLAGS} = [ "rs" ];
+                    $user{RANLIB} = ":";
+                }
             } elsif (-f "$ndk/AndroidVersion.txt") {    #"standalone toolchain"
                 my $cc = $user{CC} // "clang";
                 # One can probably argue that both clang and gcc should be