support: Use unwinder in links-dso-program-c only with libgcc_s

support: Use unwinder in links-dso-program-c only with libgcc_s

Do not build links-dso-program-c with exception (unwinding) support
if libgcc_s is not available.  In this case, the unwinder may be
part of libgcc.a or libgcc_eh.a, depending on how GCC was built.
If the unwinder is in libgcc_eh.a only, linking links-dso-program-c
failed before this change.  After this change, the exception
handling landing pad is only generated if libgcc_s available,
avoiding an undefined _Unwind_Resume (or equivalent) symbol
reference in the non-libgcc_s case.

Fixes commit ffd36cc27407003a6f9efcb9c16370e3435c5b1d ("support: Use
unwinder in links-dso-program-c only with libgcc_s") and
commit 5dfbc3c43ecc1bcfc760a032c91bb002660051bc ("support: Link
links-dso-program-c with libgcc_s only if available").