Make words() work on a list of non-strings

words() attempts to join all the elements of a list
and then resplit them to more closely match make.
But sometimes, like when calling words() on a product
variable, not all of the elements are strings. In
that case, just return the list unchanged.

Bug: 267407943
Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: I738d0c86c8935f446807cc79623f796e8cae3c01