meson: set -fno-ssa-phiopt when building bpf with gcc

There are bugs in the kernel verifier that cause legitimate code
to be rejected, disabling this optimization makes bpf programs
built with a new enough gcc work again.

Fixes https://github.com/systemd/systemd/issues/31888

meson.build

@@ -1701,6 +1701,7 @@ if conf.get('BPF_FRAMEWORK') == 1
         bpf_gcc_flags = [
                 '-std=gnu11',
                 '-fno-stack-protector',
+                '-fno-ssa-phiopt',
                 '-O2',
                 '-mcpu=v3',
                 '-mco-re',