[Sanitizers] [Bug 24151] Generalize type of offset in internal_mmap

Summary:
Using u64 as type for offset changes its value, changing starting address for map in file.
This patch solves Bug 24151, which raises issue while mapping file in mips32.

Patch by Mohit Bhakkad

Reviewers: dsanders, kcc

Subscribers: hans, llvm-commits, samsonov, nitesh.jain, sagar, bhushan, jaydeep

Differential Revision: http://reviews.llvm.org/D11588

llvm-svn: 243686