Bug 1779257 - Fix the stack walking for linux markers by calling getcontext in...

Bug 1779257 - Fix the stack walking for linux markers by calling getcontext in a valid stack frame r=mstange

See my comment on here for more context of my investigation:
https://bugzilla.mozilla.org/show_bug.cgi?id=1779257#c9

The saved context is invalid once the function that called `getcontext`
returns. We need to call the `getcontext` while the frame where we called it is
still on the stack. That's why this patch is moving the call to `getcontext` to
parent function by inlining the SyncPopulate content by using a macro instead.
This has to be a macro instead of a function because stack pointer address will
be invalid once the `Registers::SyncPopulate` returns. I tried to change this
method to inline but that didn't help either.

Differential Revision: https://phabricator.services.mozilla.com/D170133