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

[mgorny]

Summary

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.

lldb/lit/SymbolFile/Inputs/target-symbols-add-unwind.c

-void _start() {}
+int main() {}

lldb/lit/SymbolFile/target-symbols-add-unwind.test

 # TODO: When it's possible to run "image show-unwind" without a running
 # process, we can remove the unsupported line below, and hard-code an ELF
 # triple in the test.
 # UNSUPPORTED: system-windows, system-darwin
 
 # RUN: cd %T
-# RUN: %clang %S/Inputs/target-symbols-add-unwind.c -nostdlib -g \
+# RUN: %clang %S/Inputs/target-symbols-add-unwind.c -g \
 # RUN:   -fno-unwind-tables -o target-symbols-add-unwind.debug
 # RUN: llvm-objcopy --strip-debug target-symbols-add-unwind.debug \
 # RUN:   target-symbols-add-unwind.stripped
 # RUN: %lldb target-symbols-add-unwind.stripped -s %s -o quit | FileCheck %s
 
 process launch --stop-at-entry
-image show-unwind -n _start
-# CHECK-LABEL: image show-unwind -n _start
+image show-unwind -n main
+# CHECK-LABEL: image show-unwind -n main
 # CHECK-NOT: debug_frame UnwindPlan:
 
 target symbols add -s target-symbols-add-unwind.stripped target-symbols-add-unwind.debug
 # CHECK-LABEL: target symbols add
 # CHECK: symbol file {{.*}} has been added to {{.*}}
 
-image show-unwind -n _start
-# CHECK-LABEL: image show-unwind -n _start
+image show-unwind -n main
+# CHECK-LABEL: image show-unwind -n main
 # CHECK: debug_frame UnwindPlan:
 # CHECK-NEXT: This UnwindPlan originally sourced from DWARF CFI
 # CHECK-NEXT: This UnwindPlan is sourced from the compiler: yes.