[lldb] [lit/SymbolFile] Avoid -nostdlib in target-symbols-add-unwind.test

Do not use -nostdlib in target-symbols-add-unwind.test.  NetBSD uses
startup files to provide obligatory ELF notes in executables,
and therefore using -nostdlib requires providing specially tailored
input.  Otherwise, kernel rejects the result as invalid executable.

The replacement was suggested by Pavel Labath.

Differential Revision: https://reviews.llvm.org/D60648

llvm-svn: 358329