Remove usage of getcontext() on supported platforms

[benpye]

We shouldn't require the use of getcontext(), BSD registers procfs, etc. getcontext() especially is deprecated in modern POSIX standards and as such libc's such as musl do not implement it. This is a blocker for supported Alpine Linux and other distributions that use musl, part of #4210 . This would then remove the need for a different way to get the registers for each platform, though it would be architecture specific.

[benpye]

cc: @janvorli @jkotas

Does this seem a reasonable proposal? In fact, shouldn't we be able to reuse the CaptureContext function that already exists across each architecture.