src/lib: Add memory/time saving special case for ramstage caching

When caching the ramstage for suspend/resume, we copy the entire image
as it resides in RAM. The last part of that, CONFIG_HEAP_SIZE bytes, is
the heap that will be reinitialized when the ramstage is started again.

As such, copying doesn't make sense and complicates HEAP_SIZE
configuration (because it needs to fit the space-constrained cache
location) and costs time and space. Therefore, skip the heap.

Side notes:
- When building with ASAN, program.ld indicates that it will allocate
  some more space after the heap. This is not a problem, we just copy
  an ASAN-sized copy of the heap.
- Heap use is managed in src/lib/malloc with statically allocated
  variables. Because ramstage is cached before it's executed, these
  values will be reset to their compile-time default values, too.

Change-Id: I6553dc8b758196f2476af2e692c0421d0fa2b98e
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79525
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

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