Fix LLVM feature detection and rebuild with LLVM 21

Resolves: RHEL-148264

bpftool disassembler optionally uses LLVM to improve its capabilities.
Unfortunately, even if llvm-devel is installed, the feature detection of
LLVM doesn't work:

    Auto-detecting system features:
    ...                         clang-bpf-co-re: [ on  ]
    ...                                    llvm: [ OFF ]
    ...                                  libcap: [ on  ]
    ...                                  libbfd: [ on  ]

The reason is that the feature detection tries to compile a simple C++
program and link it against libLLVM which fails as LDFLAGS contains -pie
while CXXFLAGS does not contain -fpie.

Fix this by setting EXTRA_CXXFLAGS from the bpftool build.

Signed-off-by: Viktor Malik <vmalik@redhat.com>

bpftool.spec

@@ -10,7 +10,7 @@
 
 Name:           bpftool
 Version:        7.7.0
-Release:        1%{?dist}
+Release:        2%{?dist}
 Summary:        Inspection and simple manipulation of eBPF programs and maps
 
 %global libname libbpf
@@ -42,7 +42,7 @@ manipulation of eBPF programs and maps.
 # We need to use vmlinux.h from kernel-devel rather than the one from the running system
 %define kernel_version %(rpm -q --qf "%%{VERSION}-%%{RELEASE}.%%{ARCH}" kernel-devel)
 pushd tools/bpf/bpftool
-%make_build EXTRA_CFLAGS="%{build_cflags}" EXTRA_LDFLAGS="%{build_ldflags}" VMLINUX_H="/usr/src/kernels/%{kernel_version}/vmlinux.h"
+%make_build EXTRA_CFLAGS="%{build_cflags}" EXTRA_CXXFLAGS="%{build_cflags}" EXTRA_LDFLAGS="%{build_ldflags}" VMLINUX_H="/usr/src/kernels/%{kernel_version}/vmlinux.h"
 
 %install
 pushd tools/bpf/bpftool