[compiler-rt] Add a routine to specify the mode used when creating profile dirs.

Summary:
This patch introduces `llvm_profile_set_dir_mode` and `llvm_profile_get_dir_mode` to
the compiler-rt profile API. 

Originally, profile data was placed into a directory that was created with a hard-coded
mode value of 0755 (for non-win32 builds).  In certain cases, it can be helpful to create
directories with a different mode other than 0755.  This patch introduces set/get
routines to allow users to specify a desired mode.  The default remains at 0755.

Reviewers: void, probinson

Reviewed By: probinson

Subscribers: probinson, dberris, cfe-commits

Differential Revision: https://reviews.llvm.org/D49953

llvm-svn: 338456