[asan] Add __sancov_* symbols to the dynamic symbol table.

[mpividori]

Summary

I took me some time to understand the reason why "__sancov_*" symbols were not exported.
I wonder why we need an export list? Why cant we just use the visibility for this? We want to have more control on the exported symbols?

[mpividori]

Hi, this change is needed for the test added in: https://reviews.llvm.org/D29148 , when listing the symbols exported with nm -D, __sancov_default_options was not shown before this diff.

[mpividori]

@kcc do you agree with this small change?

[kcc]

LGTM

compiler-rt/trunk/lib/asan/asan.syms.extra

 __asan_*
 __lsan_*
 __ubsan_*
+__sancov_*