Trim spaces in the rule actions

When rule action contains something like
  cp $(FOO) ...
and FOO is set with
FOO := a \
 b \

the generated Ninja file constains extra spaces, making it
difficult to compare it to the same file generated by the
Starlark-based product configuration.

Bug: 181797530
Test: manual
Change-Id: I278bd8edf0f017a31c5b5115b2a38f4f663c55fc