Use the PRIu64 macro for printing a uint64_t.

Otherwise on (some) 64-bit systems, -Wformat will trigger a warning
because uint64_t is an 'unsigned long' not an 'unsigned long long'.
Consequently, PGOProfiling.c would fail to build if -Werror and
-Wformat are both enabled.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@198644 91177308-0d34-0410-b5e6-96231b3b80d8