Track reg save locations via setRegisterLocation

[am11]

Adapt the REGDISPLAY shims to the reduced llvm-libunwind patch:
setRegister only carries values (IP/SP), and save locations arrive
through setRegisterLocation. Pass pc + 1 to stepWithDwarf instead of
patching the CFI row comparison, use the upstream UnwindCursor
constructor, and call the templated findUnwindSections.

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[am11]

cc @janvorli, @filipnavara, @jkotas, this refactoring reduces the patch size we apply during llvm-libunwind updates.

When ready, please merge it without squashing.

[am11]

Tiny improvement in size MichalStrehovsky/rt-sz#268. Flow-wise:

before:

DwarfInstructions / CompactUnwinder / Unwind-EHABI          (patched)
  setRegister(reg, value, location)  <-- 3rd arg everywhere
       |                          \
       v                           v
Registers_REGDISPLAY            Registers_x86_64/arm64/...   (patched)
  pRbx = location                 + location arrays  (unused)
                                  + getRegisterLocation (unused)
                                unw_cursor_t/unw_context_t enlarged
                                unw_set_reg / _Unwind_SetGR /
                                _Unwind_VRS_Set (+pos)  (public API changed)
                                unw_get_save_loc        (unused)
DwarfParser: codeOffset <= pcoffset                          (patched)

patch: 14 files, +668 / -278

after:

DwarfInstructions / CompactUnwinder
  setRegister(reg, value)               <-- upstream, unchanged
  setSavedRegisterLocation(reg, loc)    <-- new helper
       |
       |  R has setRegisterLocation()?
       +-- yes --> Registers_REGDISPLAY::setRegisterLocation
       |             pRbx = location
       +-- no  --> no-op   (libunwind's own register classes
                            stay upstream-sized)

Unwind-EHABI (ARM32)
  __unw_set_reg_location  -->  AbstractUnwindCursor::setRegLocation
    (hidden, internal)           default no-op; ArmUnwindCursor overrides

DwarfParser: upstream "<"      StepFrame passes pc + 1 instead

patch: 10 files, +160 / -48
public headers and DwarfParser.hpp: identical to upstream

[jkotas]

/azp run runtime-nativeaot-outerloop

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[am11]

global::Runtime_134891.TestEntryPoint() is failing and highlighted as new; doesn't seem related.

[jkotas]

/ba-g unrelated test failure - opened #135604

[jkotas]

Thanks!