Fix whitespace in soong_config_set calls

In make, soong_config_set uses an $(eval) to set it's value, expanding
the value of the soong config variable _before_ evaluating the value.
Because of this, make will strip trailing whitespace like it does on
regular assignments.

Make rbcrun match this behavior.

Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: I907e85cdf50f6fac54331c0d1044f0d53bec22ed