Build with clang

This is necessary to detect the correct float16 ABI.

Also fix setting of ASMFLAGS, so -fcf-protection gets set properly.

Resolves: RHEL-8713

compiler-rt.spec

@@ -32,7 +32,7 @@
 
 Name:		compiler-rt
 Version:	%{compiler_rt_version}%{?rc_ver:~rc%{rc_ver}}%{?llvm_snapshot_version_suffix:~%{llvm_snapshot_version_suffix}}
-Release:	1%{?dist}
+Release:	2%{?dist}
 Summary:	LLVM "compiler-rt" runtime libraries
 
 License:	NCSA or MIT
@@ -52,8 +52,7 @@ Patch0:		0001-compiler-rt-Fix-FLOAT16-feature-detection.patch
 Patch100:	0001-Drop-fno-stack-protector-from-the-compiler-flags.patch
 Patch101:	fix-page-size-constant.patch
 
-BuildRequires:	gcc
-BuildRequires:	gcc-c++
+BuildRequires:	clang
 BuildRequires:	cmake
 BuildRequires:	ninja-build
 BuildRequires:	python3
@@ -86,12 +85,16 @@ ln -s %{_datadir}/llvm/cmake ../cmake
 %undefine __cmake_in_source_build
 
 # Copy CFLAGS into ASMFLAGS, so -fcf-protection is used when compiling assembly files.
-export ASMFLAGS=$CFLAGS
+export ASMFLAGS="%{build_cflags}"
 
+# RHEL 8 specific: "global toolchain clang" is not supported on RHEL 8, so explicitly enable
+# clang in cmake. This is necessary to get the correct float16 ABI.
 %cmake	-GNinja \
 	-DCMAKE_BUILD_TYPE=RelWithDebInfo \
 	-DCMAKE_MODULE_PATH=%{_libdir}/cmake/llvm \
 	-DCMAKE_SKIP_RPATH:BOOL=ON \
+	-DCMAKE_C_COMPILER=clang \
+	-DCMAKE_CXX_COMPILER=clang++ \
 	-DCOMPILER_RT_INSTALL_PATH=%{_prefix}/lib/clang/%{maj_ver} \
 	-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON \
 	\
@@ -133,6 +136,9 @@ mv %{buildroot}%{_prefix}/lib/clang/%{maj_ver}/lib/powerpc64le-redhat-linux-gnu
 #%endif
 
 %changelog
+* Fri Oct 13 2023 Nikita Popov <npopov@redhat.com> - 17.0.2-2
+- Build with clang
+
 * Wed Oct 04 2023 Nikita Popov <npopov@redhat.com> - 17.0.2-1
 - Update to LLVM 17.0.2