[msan] Fast path optimization for wrap-indirect-calls feature of MemorySanitizer.

Indirect call wrapping helps MSanDR (dynamic instrumentation companion tool
for MSan) to catch all cases where execution leaves a compiler-instrumented
module by allowing the tool to rewrite targets of indirect calls.

This change is an optimization that skips wrapping for calls when target is
inside the current module. This relies on the linker providing symbols at the
begin and end of the module code (or code + data, does not really matter).
Gold linker provides such symbols by default. GNU (BFD) linker needs a link
flag: -Wl,--defsym=__executable_start=0.

More info:
https://code.google.com/p/memory-sanitizer/wiki/MSanDR#Native_exec