Make __gcov_flush flush counters for all shared libraries

Summary:
This will make the behavior of __gcov_flush match the GCC behavior.

I would like to rename gcov_flush to llvm_gcov_flush (in case of programs linking to libraries built with different compilers), but I guess we can't for compatibility reasons.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: samsonov, vitalybuka, pcc, kcc, junbuml, glider, fhahn, eugenis, dvyukov, davidxl, srhines, chh, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48538