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Add kalloc (malloc with an error pointer), free, _kernel_irqs_disabled,
_kernel_irqs_off, _kernel_irqs_on, and a simple memcpy implementation.

Export the symbols so they're actually usable from other object files.

This adds initial support for the "long descriptor" MMU page table
format, which allows the CPU to (flexibly) use a 40-bit physical address
space.

There are still some features that need fixing (e.g. RISCOS_MapInIO
flags), and the OS doesn't yet support RAM above the 32bit limit, but
this set of changes is enough to allow for working ROMs to be produced.

Also, move MMUControlSoftCopy initialisation out of ClearWkspRAM, since
it's unrelated to whether the HAL has cleared the RAM or not.

Version 5.35, 4.79.2.2. Tagged as 'Kernel-5_35-4_79_2_2'