Suppress uninitialized-variable-is-used warning in kmp_csupport.c

The following change is needed to suppress the "variable 'retval' is used 
uninitialized whenever 'if' condition is false" warnings in runtime/src/kmp_csupport.c.
This change just initializes 'retval' to 0.
http://lists.cs.uiuc.edu/pipermail/openmp-dev/2015-June/000667.html

Patch by Jack Howarth

llvm-svn: 238954