Make lto handling similar to current rawhide

Have a --with lto_build option, where LTO is enabled by default
for everything except i686 and riscv. Use that flag to condition
whether `-DLLVM_UNITTEST_LINK_FLAGS` is passed.

Additionally, make use of the new Fat LTO functionality in rawhide
to use `-fno-lto` instead of `-Wl,-plugin-opt=O0` to save more
time linking unit tests.

build.spec.inc

@@ -193,7 +193,13 @@ cd llvm
 %ifarch x86_64
 	-DCMAKE_SHARED_LINKER_FLAGS="$LDFLAGS -Wl,-z,cet-report=error" \
 %endif
+%if %{with lto_build}
+%if 0%{?fedora} >= 41
+	-DLLVM_UNITTEST_LINK_FLAGS="-fno-lto"
+%else
 	-DLLVM_UNITTEST_LINK_FLAGS="-Wl,-plugin-opt=O0"
+%endif
+%endif
 
 # Build libLLVM.so first.  This ensures that when libLLVM.so is linking, there
 # are no other compile jobs running.  This will help reduce OOM errors on the

globals.spec.inc

@@ -9,6 +9,17 @@
 %bcond_with compat_build
 %bcond_without check
 
+# Disable LTO on x86 and riscv in order to reduce memory consumption.
+%ifarch %ix86 riscv64
+%bcond_with lto_build
+%else
+%bcond_without lto_build
+%endif
+
+%if %{without lto_build}
+%global _lto_cflags %nil
+%endif
+
 # We are building with clang for faster/lower memory LTO builds.
 # See https://docs.fedoraproject.org/en-US/packaging-guidelines/#_compiler_macros
 %global toolchain clang