Config.mk: evaluate XEN_COMPILE_ARCH and XEN_OS immediately

With GNU make 4.4, the number of execution of the command present in
these $(shell ) increased greatly. This is probably because as of make
4.4, exported variable are also added to the environment of $(shell )
construct.

So to avoid having these command been run more than necessary, we
will replace ?= by an equivalent but with immediate expansion.

Reported-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

Config.mk

@@ -19,13 +19,17 @@ or       = $(if $(strip $(1)),$(1),$(if $(strip $(2)),$(2),$(if $(strip $(3)),$(
 
 -include $(XEN_ROOT)/.config
 
-XEN_COMPILE_ARCH    ?= $(shell uname -m | sed -e s/i.86/x86_32/ \
+ifeq ($(origin XEN_COMPILE_ARCH), undefined)
+XEN_COMPILE_ARCH    := $(shell uname -m | sed -e s/i.86/x86_32/ \
                          -e s/i86pc/x86_32/ -e s/amd64/x86_64/ \
                          -e s/armv7.*/arm32/ -e s/armv8.*/arm64/ \
                          -e s/aarch64/arm64/)
+endif
 
 XEN_TARGET_ARCH     ?= $(XEN_COMPILE_ARCH)
-XEN_OS              ?= $(shell uname -s)
+ifeq ($(origin XEN_OS), undefined)
+XEN_OS              := $(shell uname -s)
+endif
 
 CONFIG_$(XEN_OS) := y