[profile] Remove useless msync when dumping gcda files

Summary:
According the mmap man page (https://man7.org/linux/man-pages/man2/mmap.2.html) is only required to precisely control updates, so we can safely remove it.
Since gcda files are dumped just before to call exec** functions, dump need to be fast.
On my computer, Firefox built with --coverage needs ~1min40 to display something and in removing msync it needs ~8s.

Reviewers: void

Subscribers: #sanitizers, marco-c, sylvestre.ledru

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D81060