UEFI target groundwork: runtime: split baremetal memory setup

[sparques]

I am back with another attempt trying to get UEFI as a target.

Summary:

This PR moves baremetal-specific memory symbol declarations and initialization out of src/runtime/baremetal.go into a separate file, src/runtime/baremetal_memory.go.

The extracted code covers:

• _heap_start / _heap_end
• _globals_start / _globals_end
• _stack_top
• the default growHeap() implementation for baremetal targets

Why:

baremetal.go currently mixes general baremetal runtime behavior with the default linker-symbol-based memory layout. Splitting the memory setup into its own file makes that boundary explicit and makes it easier for targets with nonstandard memory initialization to evolve without adding more special cases to the main baremetal runtime file.
For my purposes, eventually baremetal_memory.go will have a //go:build baremetal && !uefi or something to that effect.

Behavior:

This is intended to be a pure refactor. There should be no functional change for existing baremetal targets.

[deadprogram]

@sparques can you please rebase this against the latest dev branch? Thanks.

[deadprogram]

Thanks for your work on this @sparques seems like a good direction. Now squash/merging.