package/xen: add upstream security fix for XSA-312

Fixes the following security issue:

XSA-312: arm: a CPU may speculate past the ERET instruction

For further details, see the advisory:

https://xenbits.xenproject.org/xsa/advisory-312.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 76d56fe7690dfafbcf885f645436104a0c34e14c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/xen/xen.hash

@@ -1,3 +1,4 @@
 # Locally computed
 sha256 1c75cbe728dfabf02b7f9a17ce96ee7d202d2fd4b4689490018d3a28b63f9fa3 xen-4.12.2.tar.gz
+sha256 9b2078d448e4815c9ddc6554bf869d64412dc787b1b94830a24e47df6a9f30e7 xsa312.patch
 sha256 dba0d79260259c013c52e5d4daeaea564a2fbb9ff7fc6778c377a401ec3898de COPYING

package/xen/xen.mk

@@ -6,6 +6,8 @@
 
 XEN_VERSION = 4.12.2
 XEN_SITE = https://downloads.xenproject.org/release/xen/$(XEN_VERSION)
+XEN_PATCH = \
+	https://xenbits.xenproject.org/xsa/xsa312.patch
 XEN_LICENSE = GPL-2.0
 XEN_LICENSE_FILES = COPYING
 XEN_DEPENDENCIES = host-acpica host-python