Makefile: Fix 4.19.15 resolution

Commit 1637d5d2 ("kbuild: consolidate Clang compiler flags")
conflicts with commit 4c451dba ("ANDROID: Kbuild, LLVMLinux: allow
overriding clang target triple"). As it currently stands, CLANG_FLAGS
will not have the proper target parameter when cross compiling for
aarch64-linux-android. Move the CLANG_FLAGS definition up and use it
when checking for the proper target fallback, allowing us to properly
remove CLANG_TARGET.

Change-Id: I176146ad613bd5e6187f3421f29c2ffcc9ceebc3
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>