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

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>