Fix postgresql musl: restore llvmPackages override + -fno-lto

hardeningDisable=["lto"] is not valid on musl cross stdenv (causes
eval failure: "unsupported hardening flags"). Instead:
- Restore llvmPackages_20 override to prevent LLVM dependency
- Add -fno-lto via NIX_CFLAGS_COMPILE to disable GCC LTO
  (GCC LTO + GNU ld produces .ltrans link failures in postgresql)

Author: angerman

flake.nix

@@ -132,15 +132,27 @@
            # 2. perlSupport defaults true → musl perl lacks shared libperl.
            # 3. generic.nix switches to LLVM stdenv+bintools for LTO when
            #    GCC is used. Cross-compiled LLVM 20 OOMs on darwin builders.
-           #    The switch provides ld.lld for GCC LTO; without it, GCC LTO
-           #    with GNU ld produces broken link (undefined refs in .ltrans).
-           #    Fix: disable LTO entirely via hardeningDisable.
+           #    We override llvmPackages_20 to prevent the switch (avoids
+           #    LLVM dependency), then explicitly disable LTO with -fno-lto
+           #    since GCC LTO + GNU ld is broken for postgresql (.ltrans
+           #    link failures). Can't use hardeningDisable=["lto"] because
+           #    "lto" is not a recognized hardening flag on musl cross.
            # 4. outputChecks unconditionally reference llvmPackages.llvm.
            postgresql = (prev.postgresql.override {
              jitSupport = false;
              perlSupport = false;
-           }).overrideAttrs (_: {
-             hardeningDisable = [ "lto" ];
+             # Prevent the LTO stdenv switch: provide normal GCC-based
+             # musl stdenv as llvmPackages_20, making the switch a no-op.
+             llvmPackages_20 = prev.llvmPackages_20 // {
+               inherit (prev) stdenv;
+               bintools = prev.stdenv.cc.bintools;
+             };
+           }).overrideAttrs (old: {
+             # Explicitly disable LTO since we're using GCC + GNU ld
+             # (not the LLVM bintools the stdenv switch would provide).
+             env = (old.env or {}) // {
+               NIX_CFLAGS_COMPILE = (old.env.NIX_CFLAGS_COMPILE or "") + " -fno-lto";
+             };
              doCheck = false;
              outputChecks = {};
            });