tcg: Add --accel tcg,split-wx property

Plumb the value through to alloc_code_gen_buffer.  This is not
supported by any os or tcg backend, so for now enabling it will
result in an error.

Reviewed-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/qemu-options.hx b/qemu-options.hx
index 459c916..1698a0c 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -115,6 +115,7 @@
     "                igd-passthru=on|off (enable Xen integrated Intel graphics passthrough, default=off)\n"
     "                kernel-irqchip=on|off|split controls accelerated irqchip support (default=on)\n"
     "                kvm-shadow-mem=size of KVM shadow MMU in bytes\n"
+    "                split-wx=on|off (enable TCG split w^x mapping)\n"
     "                tb-size=n (TCG translation block cache size)\n"
     "                thread=single|multi (enable multi-threaded TCG)\n", QEMU_ARCH_ALL)
 SRST
@@ -140,6 +141,12 @@
     ``kvm-shadow-mem=size``
         Defines the size of the KVM shadow MMU.
 
+    ``split-wx=on|off``
+        Controls the use of split w^x mapping for the TCG code generation
+        buffer. Some operating systems require this to be enabled, and in
+        such a case this will default on. On other operating systems, this
+        will default off, but one may enable this for testing or debugging.
+
     ``tb-size=n``
         Controls the size (in MiB) of the TCG translation block cache.