This avoids relying on libunwind, which isn't always available,
and which can be brittle (e.g. current git master of libunwind fails, see
https://github.com/libunwind/libunwind/pull/203#issuecomment-984126066).

This allows unwinding with the EXIDX/EXTBL info which is used
normally for C++ exception handling/unwinding. This avoids needing
to keep the .so files unstripped and avoids needing libunwind to
load .debug_frame from disk instead of the already mapped
EXIDX/EXTBL.

This patch uses the dl_iterate_phdr function for finding the EXIDX
section; keeping this call within #ifdef linux to avoid breaking
someone's build, even though it probably is available on most unix
(or ELF) platforms.

Alternatively, we could add configure checks for this function.

This passes all my unwinding tests, for full ELF builds of Wine,
built with both GCC and Clang. (It also works for PE builds, where
only very few ELF bits need to be unwound.)
Signed-off-by: Martin Storsjö <martin@martin.st>