[coverage] Wrap calls to open

Bug: http://b/173448692

The coverage runtime creates files with uga+rw permissions but an
earlier umask call can restrict these and cause problems with profile
merging ('%Nm' in LLVM_PROFILE_FILE).  This change passes `--wrap,open`
to coverage builds.  The other change in this topic adds the wrapper
function to set the permissions again using `fchmod` (only for files
opened under /data/misc/trace).

Test: Manually validate permissions of files under /data/misc/trace.
Test: libprofile-clang-extras-test
Change-Id: I881474ab184d03d14c5637951aac027a363739b6