build: Don't test for KVM xsave support on ARM
build: Don't test for KVM xsave support on ARM

The current build tests for KVM unconditionally check for xsave
support. This obviously never works on ARM since xsave is
x86-specific. This changeset refactors the build tests probing for KVM
support and moves the xsave test to an x86-specific section of
is_isa_kvm_compatible().

pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5,vovojh/gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,vovojh/gem5