[sanitizer] Add strchr* to the common interceptors

Adds strchr, strchrnul, and strrchr to the common interceptors, under a new
common flag intercept_strchr.

Removes the now-duplicate strchr interceptor from asan and all 3
interceptors from tsan. Previously, asan did not intercept strchrnul, but
does now; previously, msan did not intercept strchr, strchrnul, or strrchr,
but does now.

http://reviews.llvm.org/D18329

Patch by Derek Bruening!

llvm-svn: 263992