perl: generate Errno.pm on the wasm cross target so use Errno works (kd-gtxa)

[brandonpayton]

Problem (kd-gtxa)

On the wasm cross target, Perl shipped without Errno.pm, so use Errno failed
("Can't locate Errno.pm") and any module needing errno constants broke. During
make, ext/Errno/Errno_pm.PL died: "No error definitions found at
Errno_pm.PL line 208."

Root cause

Errno_pm.PL::get_files() discovers which headers define the E* constants by
preprocessing #include <errno.h> and scanning the cpp output for
# <line> "file" linemarkers, then greps those headers for #define E*.
perl-cross defines cpp/cpprun/cppstdin as "$cc -E -P" (cnf/configure_tool.sh),
and -P suppresses linemarkers — so the scan discovered zero headers,
collected no constants, and Errno_pm.PL died. Not an include-path problem: the
SDK cc wrapper always injects --sysroot, and the constants are plain
#define E* <int> in the sysroot (musl arch/generic bits/errno.h, 134 of them).
They were simply never discovered.

Confirmed directly: wasm32posix-cc -E errno.c emits 13 linemarkers pointing at
sysroot/include/bits/errno.h; -E -P emits 0.

Fix

build-perl.sh patches get_files() to fall back to the sysroot errno headers
($WASM_POSIX_SYSROOT/include/{errno.h,bits/errno.h}) when linemarker discovery
yields nothing — surgical (zero blast radius on the configure probes that -P
serves), fallback-only (upstream discovery is untouched where it works), and
fail-loud (aborts if the anchor moves). Errno.pm is added to the runtime
post-check so a regression fails the build. Revision bumped 2→3 (perl-runtime.zip
now ships Errno.pm; perl.wasm is byte-unchanged — Errno is pure-perl, not
linked in). The formula installs it automatically (it unzips lib/*).

Verification

• Reproduced on the unmodified perl: ship a usable core-module runtime (generated files + static XS) so File::Spec/POSIX load on Kandelo #821 recipe: Errno_pm.PL line 208 → Errno.pm
  absent from privlib + perl-runtime.zip.
• Clean fresh-source rebuild (exercises the recipe patch path): exit 0,
  Errno.pm generated with 134 constants at exact musl values and staged into
  perl-runtime.zip.
• Node smoke (demo/errno-smoke.ts): PERL_ERRNO_SMOKE_PASS — 16/16
  (use Errno loads; EPERM/ENOENT/…/EINVAL match musl; ERRNO_COUNT=134; %! tie).
• Browser smoke (demo/errno-browser-smoke.ts): committed + ready; skips
  with reason where the browser bundle/playwright are absent. Host-agnostic:
  perl.wasm is byte-identical to the interpreter perl: run on the default/unset locale without a startup panic (kd-dvph) #822 browser-verified, and the
  browser perl-vfs image is built from perl-src/lib (now containing Errno.pm).
  Full browser acceptance tracked by kd-yuef.

Stacking

Stacked on #821 (kd-k7zy) — the Errno failure only exists on that recipe
(origin/main's build-perl.sh stops at make perl and never builds ext/Errno).
Base is the #821 branch; retarget to main after #821 merges.

🤖 Generated with Claude Code