[profile] Support merge pool size >= 10

The executable acquires an advisory record lock (`fcntl(fd, F_SETLKW, *)`) on a profile file.
Merge pool size >= 10 may be beneficial when the concurrency is large.

Also fix a small problem about snprintf. It can cause the filename to be truncated after %m.

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D71970