fix: support musl on Linux

[jpnurmi]

This PR includes:

• libunwind -based unwinder by @supervacuus (fix/use_libunwind_as_macos_unwinder)
• build system changes to select SENTRY_WITH_LIBUNWIND for musl
• crashpad update: fix: compilation errors with musl crashpad#125
• ci changes to test with musl and libunwind

Closes #1232

[bruno-garcia]

PR title has:

• crashpad update: fix: support musl on Linux #1233

Did you mean to link to:

• feat: add CrashpadClient::Add/RemoveAttachment() for Windows and Linux crashpad#124

[jpnurmi]

PR title has:

• crashpad update: fix: support musl on Linux #1233

Did you mean to link to:

• feat: add CrashpadClient::Add/RemoveAttachment() for Windows and Linux crashpad#124

Oops, thanks! It was meant to link to:

• fix: compilation errors with musl crashpad#125

[supervacuus]

If only commit suggestions could have a clang-format pass 😅

[jpnurmi]

No worries :)