extend syscall minijail for clang code coverage

cutover to clang-based code coverage uses extra system call (ftruncate)
when writing coverage data. exposed while generating coverage for media.extractor.

Bug: 160917521
Test: build, boot, kill -37 extractors for {arm,x86}x{32,64}
Change-Id: I555b168c7aab43caa590df951861b92e8fa14bc3