Handle leading underscores in update_cc_test_checks.py

For some ABIs `update_cc_test_checks.py` is unable to generate
tests because of the mismatch between the mangled function names
reported by clang's `-asd-dump` and the function names in LLVM IR.

This patch fixes it by striping the leading underscore from
the mangled name for global functions if the data layout string says
the have one.

Author: momchil-velikov