meson.build: Disable -fzero-call-used-regs on OpenBSD

QEMU currently does not work on OpenBSD since the -fzero-call-used-regs
option that we added to meson.build recently does not work with the
"retguard" extension from OpenBSD's Clang. Thus let's disable the
-fzero-call-used-regs here until there's a better solution available.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2278
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240411120819.56417-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -562,7 +562,11 @@ hardening_flags = [
 #
 # NB: Clang 17 is broken and SEGVs
 # https://github.com/llvm/llvm-project/issues/75168
-if cc.compiles('extern struct { void (*cb)(void); } s; void f(void) { s.cb(); }',
+#
+# NB2: This clashes with the "retguard" extension of OpenBSD's Clang
+# https://gitlab.com/qemu-project/qemu/-/issues/2278
+if host_os != 'openbsd' and \
+   cc.compiles('extern struct { void (*cb)(void); } s; void f(void) { s.cb(); }',
                name: '-fzero-call-used-regs=used-gpr',
                args: ['-O2', '-fzero-call-used-regs=used-gpr'])
     hardening_flags += '-fzero-call-used-regs=used-gpr'