Set LOCAL_MULTLIB := first for all LLVM host builds

Set LOCAL_MULTLIB := first for all LLVM host builds

The default multlib setting for host libraries is going to change from
"both" to "first".  This breaks LLVM host compiles on 64-bit when
FORCE_BUILD_LLVM_COMPONENTS, because LLVMHello tries to compile for
both 64-bit and 32-bit, the 32-bit version tries to link against
32-bit libLLVM, but prebuilts/sdk/tools/* only provides a 64-bit
version of libLLVM.so.

When compiling for a 32-bit host we always set
FORCE_BUILD_LLVM_COMPONENTS=true, so the easy fix is to force 64-bit
host builds to only build LLVM components for 64-bit by setting
LOCAL_MULTILIB := first in llvm-host-build.mk.

Change-Id: Ia17c045649b59b90bdc06d4cbe03f2542bf42371