Use signals for activation injection on macOS (#46657)

Use signals for activation injection on macOS (#46657)

This change moves macOS activation injection to the signal plan like it
works on other Unix platforms. The reason is that the activation injection
using thread suspension and thread redirection with helper frame can
collide with signal handlers on the same thread and result in a corrupted
stack frame. The issue can be reproduced by sending signals to the
.NET process from some other process while the .NET process is doing
a lot of GCs.