Build libomp with gcc again

@jchecahi@redhat.com Noticed that during the update from LLVM 14 to LLVM 15 several `__kmpc_atomic_*` functions got removed. According to @npopov@redhat.com this is a side-effect of building libomp with clang instead of gcc. That's why we decided to build libomp with gcc again instead of clang. libomp will only be used as a library by programs created with clang or gcc, hence the chosen compiler to build libomp itself matters less.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1988155

Resolves: rhbz#2118319

libomp.spec

@@ -1,4 +1,4 @@
-%global toolchain clang
+%global toolchain gcc
 
 %global maj_ver 15
 %global libomp_version %{maj_ver}.0.1
@@ -14,7 +14,7 @@
 
 Name: libomp
 Version: %{libomp_version}%{?rc_ver:~rc%{rc_ver}}
-Release: 1%{?dist}
+Release: 2%{?dist}
 Summary: OpenMP runtime for clang
 
 License: NCSA
@@ -75,6 +75,11 @@ OpenMP regression tests
 
 %build
 
+%if "%toolchain" == "gcc"
+# Building openmp with LTO fails with GCC but works with Clang
+%define _lto_cflags %{nil}
+%endif
+
 %cmake	-GNinja \
 	-DLIBOMP_INSTALL_ALIASES=OFF \
 	-DCMAKE_MODULE_PATH=%{_libdir}/cmake/llvm \
@@ -160,6 +165,9 @@ rm -rf %{buildroot}%{_libdir}/libarcher_static.a
 %{_libexecdir}/tests/libomp/
 
 %changelog
+* Fri Oct 28 2022 Konrad Kleine <kkleine@redhat.com> - 15.0.1-2
+- Build libomp runtime library with gcc
+
 * Thu Sep 29 2022 Konrad Kleine <kkleine@redhat.com> - 15.0.1-1
 - 15.0.1 Release