build: Support SYSROOT_DIR in native_sim and clang

When running naive sim builds using clang, respect user passed
SYSROOT_DIR values. Additionally, allow overriding the archive tool
when calling Make for the native simulator.

Add the sysroot (if available) to the native_simulator target and
TOOLCHAIN_C_FLAGS and TOOLCHAIN_LD_FLAGS. Additionally pass CMAKE_AR
to NSI_AR.

Signed-off-by: Yuval Peress <peress@google.com>