Fix `BINPREFIX` in builder-funcs.sh.

The GCC `BINPREFIX` was being clobbered when using Clang, but
`BINPREFIX` must contain binutils (specifically ar).

Stop clobbering BINPREFIX, and fix the call to
`builder_set_binprefix_llvm` so it actually passes both arguments.

Bug: http://b/26218424
Change-Id: I4d4c2254b4c388f5484db1b0dcebb1ca566a130b
diff --git a/build/tools/builder-funcs.sh b/build/tools/builder-funcs.sh
index fe05f20..71d5bbf 100644
--- a/build/tools/builder-funcs.sh
+++ b/build/tools/builder-funcs.sh
@@ -543,7 +543,7 @@
     if [ -n "$LLVM_VERSION" ]; then
         GCC_TOOLCHAIN=`dirname $BINPREFIX`
         GCC_TOOLCHAIN=`dirname $GCC_TOOLCHAIN`
-        BINPREFIX=$(get_llvm_toolchain_binprefix $LLVM_VERSION $TAG)
+        LLVM_BINPREFIX=$(get_llvm_toolchain_binprefix $LLVM_VERSION $TAG)
     fi
 
     if [ -z "$PLATFORM" ]; then
@@ -570,7 +570,7 @@
     if [ -z "$LLVM_VERSION" ]; then
         builder_set_binprefix "$BINPREFIX"
     else
-        builder_set_binprefix_llvm "$BINPREFIX"
+        builder_set_binprefix_llvm "$LLVM_BINPREFIX" "$BINPREFIX"
         case $ABI in
             armeabi)
                 LLVM_TRIPLE=armv5te-none-linux-androideabi