cpu: Fix build for MacOS.

* The value of build environment variable KVM_ISA is serialized into
  the generated file `kvm_isa.hh'. This value should be a string, but on
  hosts where the KVM headers are not available, the default `None` is
  inserted. Changed the default value to the string `""` in this case.

* Added missing include for `std::array`.

Change-Id: I651122cc46fc9c0757f592b05f4b4cab285cb91f
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57889
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/kvm/SConsopts

@@ -40,7 +40,9 @@ with gem5_scons.Configure(main) as conf:
     # we rely on exists since version 2.6.36 of the kernel, but somehow
     # the KVM_API_VERSION does not reflect the change. We test for one of
     # the types as a fall back.
-    conf.env['KVM_ISA'] = None
+    # The default value of KVM_ISA should serialize to a string in the
+    # C++ header and test False in Scons/Python.
+    conf.env['KVM_ISA'] = ''
     if not conf.CheckHeader('linux/kvm.h', '<>'):
         print("Info: Compatible header file <linux/kvm.h> not found, "
               "disabling KVM support.")

src/cpu/static_inst.hh

@@ -42,6 +42,7 @@
 #ifndef __CPU_STATIC_INST_HH__
 #define __CPU_STATIC_INST_HH__
 
+#include <array>
 #include <bitset>
 #include <cstdint>
 #include <memory>