testsuite, Darwin : Fix the asan/strncpy-overflow-1 test.

testsuite, Darwin : Fix the asan/strncpy-overflow-1 test.

1. To be more compatible with Linux, Darwin testcases that include
string.h should set _FORTIFY_SOURCE=0 since, otherwise, it will be
defaulted on and the _chk versions of the string builtins will be
used.  This testcase fails otherwise because there's no convenient
way to disable the _chk builtins.

2. The system tool that handles symbolization (atos) is not reliable
with GCC's DWARF-2 output but, fortunately, all the platform
versions that support the current sanitizers are able to handle
dwarf-3 for this testcase.

gcc/testsuite/ChangeLog:

	* c-c++-common/asan/strncpy-overflow-1.c: Add _FORTIFY_SOURCE=0 and
	-gdwarf-3 to the command line options. Adjust the expected line
	numbers for the revised options header.