cmake: Fix FindLlvmLld usage of unimported package

The find_package_handle_standard_args function is used in FindLlvmLld
without being imported. This may cause FindLlvmLld to fail
non-determistically based on if it's evaluated in a CMake build/process
before find_package_handle_standard_args is included.

We should include what we use. Explicitly include
find_package_handle_standard_args.

Signed-off-by: Aaron Massey <aaronmassey@google.com>