[ARM64 POC - OLD] Cygwin: obtain stack base on AArch64

[crutkas]

Caution

OLD ARM64 POC - REFERENCE ONLY

Do not merge. Do not use as a PR base. Do not add to a new stack. Do not consume its build artifacts.
vNext epoch: 2026-08-31-v1
Classification: POC-SUPERSEDED
Replacement: pending

Summary

• backport accepted Cygwin commit e97c157432985faa5a26a900a8dfb7a388a74857 so __getreent() and cygload obtain the stack base from AArch64 x18
• extend the digest-pinned real aarch64-pc-cygwin compiler check to assemble the affected inline __getreent() path
• preserve the existing x86_64 path and the aarch64-pc-cygwin target spelling

Stack

This PR is intentionally based on crutkas-fix-aarch64-configure-blocker, the head branch of upstream draft PR msys2/msys2-runtime#359. It will be retargeted/submitted upstream after PR msys2#359; PR msys2#359 is unchanged.

Validation

• accepted backport has the same stable patch ID as Cygwin e97c1574
• digest-pinned ARM64 compiler job passes both the register-context check and the new assembled __getreent check
• one-time full AArch64 build probe compiles newlib/libm/math/k_standard.c successfully and reaches the next blocker: newlib/libm/ld/e_acoshl.c:41:2: error: #error "Unsupported long double format"
• existing workflow continues to exercise x86_64 configure/build/install

Branch drift

The cherry-pick applied without conflicts. Existing MSYS2-specific cygload.cc DLL naming remains unchanged; only the accepted ARM64 stack-base hunk was added. The later non-contiguous signal/SEH series beginning around f38c75a is deliberately excluded.