[esan] Extend shadow mapping to cover low libraries

Summary:
Adds support for app libraries starting slightly below 0x7f00'00000000 (the
mmap ASLR range extends down to 0x7efb'f8000000 for reasonable stack
limits) by switching to a shadow mapping offset of 0x1300'00000000.

Reviewers: aizatsky

Subscribers: vitalybuka, zhaoqin, kcc, eugenis, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D20479

llvm-svn: 270255