This detects an overflowed flash partition, such as
```
1452105 bytes used, -10313 bytes free in flash firmware space out of 1441792 bytes (1408.0kB).
444428 bytes used, 1652724 bytes free in ram for stack and heap out of 2097152 bytes (2048.0kB).
```
on a metro esp32-s2 built with debugging.
From the change:
// xtensa has more registers than an instruction can address. The 16 that
// can be addressed are called the "window". When a function is called or
// returns the window rotates. This allows for more efficient function calls
// because ram doesn't need to be used. It's only used if the window wraps
// around onto itself. At that point values are "spilled" to empty spots in
// the stack that were set aside. When the window rotates back around (on
// function return), the values are restored into the register from ram.
// So, in order to read the values in the stack scan we must make sure all
// of the register values we care about have been spilled to RAM. Luckily,
// there is a HAL call to do it. There is a bit of a race condition here
// because the register value could change after it's been restored but that
// is unlikely to happen with a heap pointer while we do a GC.
Fixes#2907
