Change Unix HW exception unwinding to not to go through signal trampoline (dotnet...

Change Unix HW exception unwinding to not to go through signal trampoline (dotnet/coreclr#5140)

This change modifies the HW exception handling on Unix so that it doesn't unwind
from the context of the DispatchManagedException through the signal trampoline
to the actual location of the exception and uses the exception's context instead.
This fixes problem that some target systems like ARM Linux have with unwinding
through that trampoline.

Commit migrated from https://github.com/dotnet/coreclr/commit/37d2c7bed790af03173bb66aeff37796cb25eb66