[Flang] Fixed installation permission of the "binary" flang

Under current configuration, the permission of `flang` after installation is 700.
This could bring a problem for system administrators who build and install flang
for other users, which only the user who builds LLVM can execute it, and others
can not. In this patch, the explicit permission setting in the `install` command
is removed, and let CMake determine what perssion to be used like other components.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D87783