vms-e11: imgact EM_ALPHA backend + activation proof

[baron-3dl]

Summary

• Adds src/imgact/arch/alpha/ (imgact_arch.h + start.S), mirroring the x86_64/aarch64 IMGACT.EXE backends and the #elif defined(__alpha__) branch in imgact.c (rd vms-e11, Alpha[A2], epic vms-8954).
• A cross-built, freestanding static-PIE Alpha OVMX image (PT_INTERP=IMGACT.EXE, DT_NEEDED on a shareable image) activates and runs under qemu-alpha, printing IMGACT-TEST: PASS (r1=42 r2=42 r3=7 r4=0), exit 0. The missing-shareable failure path correctly yields %IMGACT-F-IMGNOTFND + nonzero exit.
• x86_64/aarch64 backends re-verified unchanged (both proof harnesses still PASS, identical relocation sets and output).

EM_ machine value (oracle)

readelf -h on a real alpha-linux-gnu-gcc-built object reports Machine: Alpha; the toolchain's own <elf.h> (both /usr/include/elf.h and the alpha sysroot's) defines #define EM_ALPHA 0x9026. imgact.c selects the arch header at compile time via #if defined(__alpha__) (same pattern the x86_64/aarch64 backends already use — CMake's CMAKE_SYSTEM_PROCESSOR branch is the analogous compile-time selector), so no runtime e_machine check was needed or added.

Real Alpha ABI wrinkles found (all #ifdef/#if defined(__alpha__)-guarded; x86_64/aarch64 behavior byte-identical)

1. MAP_ANONYMOUS is 0x10 on Alpha, not the generic 0x20 other archs use (confirmed against alpha-linux-gnu's <asm/mman.h>; empirically, a plain 0x20 anonymous mmap() returned -EBADF).
2. The SysV .hash section uses 8-byte (Elf64_Xword) fields on Alpha, not the 4-byte fields every other 64-bit port (including aarch64/x86_64) uses — confirmed via sh_entsize==8 and empirically (the 4-byte-field read walked a garbage bucket/chain index into an out-of-bounds symtab access → SIGSEGV under qemu-alpha). obj_find()/struct obj generalized on a new imgact_hashword_t typedef, gated on IMGACT_HASH_XWORD (only alpha's arch header defines it).
3. Alpha has no TLSDESC (alpha-linux-gnu-gcc -mtls-dialect=desc is rejected as unrecognized). Its only TLS model is the classic General Dynamic one: a JMP_SLOT call to __tls_get_addr(&tls_index), with DTPMOD64/DTPREL64 relocations filling the two-word tls_index. Implemented as a new IMGACT builtin (resolving directly against IMGACT's own g_objs[] module table — no real DTV needed) plus two new apply_rela() cases, both guarded on IMGACT_R_TLS_DTPMOD. The existing IMGACT_R_TLSDESC case is now itself guarded on #ifdef IMGACT_R_TLSDESC, since Alpha's arch header deliberately doesn't define it.
4. (Not Alpha-specific in cause, but only reachable once Alpha's DT_HASH lookup was exercised for the first time: the hash-bucket modulo used plain %, which GCC lowers to a libgcc __remqu call on Alpha — unavailable in this -nostdlib build, left as a dangling GLOB_DAT relocation IMGACT's RELATIVE-only self_relocate() never fills. Replaced with a small standard-calling-convention umod64(): software binary long division on Alpha, a plain % pass-through everywhere else.)

Test plan

• src/imgact/test/run_test_alpha.sh (containerized via tools/cross-alpha/, qemu-alpha user-mode): activation + failure-path PASS.
• src/imgact/test/run_test_x86_64.sh (native x86_64 host): unchanged PASS.
• aarch64 cross build + qemu-aarch64: unchanged PASS (same relocation set, same PASS output).

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com