Correct pkg-config include path when cross-compiling

Prior to this commit, the include path for some libraries found through
pkg-config when cross compiling is not correct:

    $ grep include CMakeCache.txt
    blas_INCLUDEDIR:INTERNAL=/usr/include/x86_64-linux-gnu
    ...

This is because we did not tell pkg-config to search in the right path.
Since for cross-compilation, we install packages in parallel in the
system, override the primary directory where the .pc files located, in
the cross-compilation toolchain file for cmake.

Closes: #746

cmake_targets/cross-arm.cmake

@@ -4,6 +4,8 @@ set(CMAKE_SYSTEM_PROCESSOR arm)
 set(CMAKE_C_COMPILER   /usr/bin/aarch64-linux-gnu-gcc-11)
 set(CMAKE_CXX_COMPILER /usr/bin/aarch64-linux-gnu-g++-11)
 
+set(ENV{PKG_CONFIG_LIBDIR} "/usr/lib/aarch64-linux-gnu/pkgconfig")
+
 set(CROSS_COMPILE 1)
 set(bnProc_gen_128_DIR    ${CMAKE_CURRENT_BINARY_DIR}/${NATIVE_DIR}) # /../build)
 set(bnProc_gen_avx2_DIR   ${CMAKE_CURRENT_BINARY_DIR}/${NATIVE_DIR}) # /../build)