tsan: mark cur_thread_placeholder definition as initial-exec

See https://sourceware.org/bugzilla/show_bug.cgi?id=20805 and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78294 for context.
Previously we marked only declaration as initial-exec. But compilers treat
initial-exec attribute somewhat differently. Mark definition as well.

llvm-svn: 287629