Building for ARM with libunwind available has been broken
since 89f3c597, due to
references to raise_func_trampoline that were left behind.

In Linux builds, libunwind isn't practically needed since
a27b202a (which implemented an
internal EHABI unwinder). That unwinder currently only supports
Linux, due to relying on dl_iterate_phdr, but if necessary, we
could also try to detect support for dl_iterate_phdr in configure
for other OSes.
Signed-off-by: Martin Storsjö <martin@martin.st>