Subject: [PATCH] alt/glibc.spec: enable multiarch (IFUNC) for i586 by building as i686

On ALT p11, i586-glibc-core is built for the i586 target with multiarch
disabled, so memcpy/memmove/memset/strlen/strcmp/memcmp and the other string
functions are the old generic i586 code (plain FUNC symbols, no IFUNC).
The i586 memcpy reads the destination buffer before writing it, which makes
32-bit applications that copy large amounts of data (Steam games, e.g.
Half-Life 2 with Mesa/radeonsi) run 6-9 times slower than with the Fedora or
freedesktop-sdk 32-bit glibc (20-30 FPS vs 180 FPS on the same machine).

The i386 IFUNC variants are in sysdeps/i386/i686/multiarch, which is only used
when configured for an i686 target; simply adding --enable-multi-arch for
%ix86 fails to link ("undefined reference to __memcmp_ia32").

Enable multiarch on %ix86 and configure it with --build/--host=i686-alt-linux.
A test build of glibc-2.38.0.223.f053ff-alt1.p11.1 in hasher for i586 succeeds
and exports 48 IFUNC symbols (same set as the freedesktop-sdk 26.08 i386 libc).
Note: this makes the library require i686 (cmov) CPUs; real i586-only
processors would no longer be supported by this package.

---
--- a/alt/glibc.spec	2026-10-09 15:28:33.154117397 +0500
+++ b/alt/glibc.spec	2026-10-09 15:28:33.164537673 +0500
@@ -24,7 +24,9 @@
 %def_with locales
 %def_enable memusagestat
 
-%ifarch x86_64
+# IFUNC variants (SSE2/AVX) of memcpy/memmove/memset/strlen/strcmp/... live in
+# sysdeps/i386/i686/multiarch, so multiarch on %ix86 requires an i686 target.
+%ifarch x86_64 %ix86
 %def_enable multiarch
 %else
 %def_disable multiarch
@@ -361,6 +363,9 @@
 %global optflags_lto %nil
 
 %configure \
+%ifarch %ix86
+	--build=i686-alt-linux --host=i686-alt-linux \
+%endif
 	--disable-crypt \
 	--disable-profile \
 	--enable-bind-now \