stacktrace_from_exception on non-x86

I submitted the below to the mailing list, but I wanted to re-iterate it here for more eyes. https://lists.boost.org/Archives/boost/2024/04/256604.php

This feature is guarded by BOOST_STACKTRACE_ALWAYS_STORE_IN_PADDING, but that is only enabled on [x86](https://github.com/boostorg/stacktrace/blob/a33e198cf660c213e700d6f050a4641b74ea880a/src/exception_headers.h#L9-L13).
This means that

```console
./bootstrap.sh --with-toolset=gcc --with-libraries=stacktrace
./b2 install threading=multi toolset=gcc
```

always fails to build on non-x86 platforms. There are three current workarounds:

1. set boost.stacktrace.from_exception=off
2. set BOOST_STACKTRACE_LIBCXX_RUNTIME_MAY_CAUSE_MEMORY_LEAK
3. build with threading=single

(1) prevents me from using this feature. (2) shouldn't be needed on it's face because I'm using gcc/libstdc++. (3) affects more than stacktrace.

Is this an incidental error, or is there a reason I wasn't able to deduce as to why this feature can't be used on non-x86+libstdc++?

It seems like the check for libc++ on non-x86 can't be entirely done at compile time. I definitely understand wanting to alert users to the dangers of using this feature, but there also doesn't seem to be a good way of detecting the presence of libc++ at compile time (hence the need for [is_libcpp_runtime](https://github.com/boostorg/stacktrace/blob/a33e198cf660c213e700d6f050a4641b74ea880a/src/from_exception.cpp#L81)).