configure: move -mcx16 flag out of CPU_CFLAGS

The point of CPU_CFLAGS is really just to select the appropriate multilib,
for example for library linking tests, and -mcx16 is not needed for
that purpose.

Furthermore, if -mcx16 is part of QEMU's choice of a basic x86_64
instruction set, it should be applied to cross-compiled x86_64 code too;
it is plausible that tests/tcg would want to cover cmpxchg16b as well,
for example.  In the end this makes just as much sense as a per sub-build
tweak, so move the flag to meson.build and cross_cc_cflags_x86_64.

This leaves out contrib/plugins, which would fail when attempting to use
__sync_val_compare_and_swap_16 (note it does not do yet); while minor,
this *is* a disadvantage of this change.  But building contrib/plugins
with a Makefile instead of meson.build is something self-inflicted just
for the sake of showing that it can be done, and if this kind of papercut
started becoming a problem we could make the directory part of the meson
build.  Until then, we can live with the limitation.

Signed-off-by: Artyom Kunakovsky <artyomkunakovsky@gmail.com>
Message-ID: <20240523051118.29367-1-artyomkunakovsky@gmail.com>
[rewrite commit message, remove from configure. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -512,10 +512,7 @@ case "$cpu" in
     cpu="x86_64"
     host_arch=x86_64
     linux_arch=x86
-    # ??? Only extremely old AMD cpus do not have cmpxchg16b.
-    # If we truly care, we should simply detect this case at
-    # runtime and generate the fallback to serial emulation.
-    CPU_CFLAGS="-m64 -mcx16"
+    CPU_CFLAGS="-m64"
     ;;
 esac
 
@@ -1203,7 +1200,7 @@ fi
 : ${cross_cc_cflags_sparc64="-m64 -mcpu=ultrasparc"}
 : ${cross_cc_sparc="$cross_cc_sparc64"}
 : ${cross_cc_cflags_sparc="-m32 -mcpu=supersparc"}
-: ${cross_cc_cflags_x86_64="-m64"}
+: ${cross_cc_cflags_x86_64="-m64 -mcx16"}
 
 compute_target_variable() {
   eval "$2="

meson.build

@@ -336,6 +336,13 @@ if host_arch == 'i386' and not cc.links('''
   qemu_common_flags = ['-march=i486'] + qemu_common_flags
 endif
 
+# ??? Only extremely old AMD cpus do not have cmpxchg16b.
+# If we truly care, we should simply detect this case at
+# runtime and generate the fallback to serial emulation.
+if host_arch == 'x86_64'
+  qemu_common_flags = ['-mcx16'] + qemu_common_flags
+endif
+
 if get_option('prefer_static')
   qemu_ldflags += get_option('b_pie') ? '-static-pie' : '-static'
 endif