soc/intel_adsp: Linkage cleanup

Syntax beautification and general simplification pass:

+ Remove some spurious padding around the _data_end symbol.  That's a
  Zephyr symbol and should reflect the actual end of the data section in
  bytes.

+ Squash a warning that crept in where the linker doesn't like
  changing cache mapping and padding at one time, clarify in docs.

+ Use the pre-existing-but-heretofore-unknown-to-me Zephyr
  debug-sections.ld include file instead of putting all the DWARF
  sections in by hand.

+ Move the .xt* sections to explicit zero addresses, since that's what
  debug-sections.ld does for its output and if I don't they'll end up
  with funny values.

+ Use Zephyr brace-on-same-line style consistently

+ Remove cargo-cult noop patterns like ALIGN(4) and ABSOLUTE(.) (none
  of these sections get furthur relocated!)

+ Clean up the SEGSTART_* API so that it doesn't need to have
  CONFIG_KERNEL_COHERENCE guards.

+ Remove a few unused/legacy symbol exports ("end", "__stack") so as
  not to pollute the namespace.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>