pvshim: make PV shim build selectable from configure

So a user can decide whether to compile a PV shim as part of the tools
build. Note that the default behavior is preserved, which is to build
a PV shim when the target or host (if target is unset) architecture is
64bit x86.

Requested-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ wei: run autogen.s ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>

config/Tools.mk.in

@@ -75,3 +75,5 @@ TINFO_LIBS          := @TINFO_LIBS@
 ARGP_LDFLAGS        := @argp_ldflags@
 
 FILE_OFFSET_BITS    := @FILE_OFFSET_BITS@
+
+CONFIG_PV_SHIM      := @pvshim@