skip data sections when looking for users of static locals

It's possible for a static local variable's data section to have
a relocation which refers to the variable symbol itself.  Fix the logic
which searches for the user of a static local variable by only looking
in text sections (i.e. functions).

Fixes #411.
add another user of is_text_section
Merge pull request #413 from jpoimboe/static-fix

skip data sections when looking for users of static locals