[buildbot] Adjust gemmlowp_INSTALL_LIBDIR based on CMake's choices (google#468)

Some of our servers default installation to `lib64` instead of `lib`.
This patch asks CMakeCache which one did it use.

---------

Signed-off-by: Peter Rong <PeterRong@meta.com>

Author: DataCorrupted

buildbot/build_tflite.sh

@@ -120,7 +120,8 @@ git clone --filter=tree:0 --no-checkout ${GEMMLOWP_REPOSITORY} gemmlowp/src/gemm
 git -C gemmlowp/src/gemmlowp checkout ${GEMMLOWP_TAG}
 cmake -GNinja -S gemmlowp/src/gemmlowp/contrib -B gemmlowp/src/gemmlowp-build \
   -DCMAKE_INSTALL_PREFIX:PATH=${PWD}/gemmlowp \
-  -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON
+  -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON   \
+  -DCMAKE_INSTALL_LIBDIR=lib
 ninja -C gemmlowp/src/gemmlowp-build install
 
 # ml_dtypes