[esan] Add custom flag support

Summary:
Adds custom flag support to EfficiencySanitizer's runtime library.

Adds an initial flag cache_line_size which will be used by multiple tools.

Reviewers: aizatsky, vitalybuka

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

Differential Revision: http://reviews.llvm.org/D20478

llvm-svn: 270256