[llvm-readobj] - An attemp to fix BB after r369191.

Few BB failed with the following error:

Command Output (stderr):
--
/home/buildbots/ppc64be-clang-lnt-test/clang-ppc64be-lnt/llvm/test/tools/llvm-readobj/stack-sizes.test:263:19: error: BADSECTION-OUT: expected string not found in input
# BADSECTION-OUT: 8 ?
                  ^
<stdin>:4:1: note: scanning from here

^

It doesn't reproduce on ubuntu/windows I have. Also, seems many of the bots
are happy too.

This slightly reorders the code to make fouts().flush() call earlier,
like it was before the r369191.

llvm-svn: 369192