[TCWG-614] Range Thunks
Finally managed to get some review on the entirety of the Range Thunks
patches. Have reorganised the patches and wrote some documentation to
make it easier to review. Responded to all review comments so far.

A long tail of frustration.

Managed to get a hello world semi-hosting test case running on the
latest QEMU running newlib-nano. This was much more difficult than I
expected due to:
- The semihosting startup code of newlib does a semi-hosting call for
top of memory regardless of whether the heap and stack location have
been identified in a linker-script.
- The QEMU semihosting response to top of memory is not helpful
leaving the stack location in an invalid memory location (latest QEMU
requires emulation of a board and not a generic machine)
- QEMU doesn't data-abort when writing to a stack location, so my
return address is helpfully read back as 0x0
Worked around by providing a large enough memory size to QEMU that the
semihosting call for top of memory returns 0, allowing newlib to use
the values in the linker script. Will probably need to spend some time
to write my own startup code that just uses the linker script for the
heap and stack.

Building compiler-rt for v6-m and v7-m has been much more difficult
than I expected as well. I've managed to find a configuration that
works, although it relies on some experimental work in moving
compiler-rt to the runtimes directory.

Plans for next week:
- LLD is top priority
- Get testing for compiler-rt working via qemu on v7-m, the recipe
that works for build does not support testing. I've got to either
extract the cmake magic flags or find a way to plumb through the runes
that make the tests work to the recipe I've got.
- On holiday Thursday, Friday and the following Monday
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