HostCross is an attribute of a Target, not OsType

A host target is considered as being cross-compiled when the target
can't run natively on the build machine. For example, linux_glibc/x86_64
is a non-cross target on a standard x86/Linux machine, but is a cross
host on Mac. Previously, whether cross or not was a static attribute of
an OsType. For example, Windows was always considered as cross host,
while linux_bionic was not. This becomes a problem when we support more
host targets like linux_bionic/arm64 which should be cross-host on
standard x86/Linux machines.

This change removes HostCross from the OsClass type and instead adds a
property HostCross to the Target type. When a target is being added, it
is initialized to true when the target can't run natively on the current
build machine.

Bug: 168086242
Test: m
Change-Id: Ic37c8db918873ddf324c86b12b5412952b0f2be2
diff --git a/android/prebuilt_test.go b/android/prebuilt_test.go
index 6c3cd9e..854395e 100644
--- a/android/prebuilt_test.go
+++ b/android/prebuilt_test.go
@@ -285,7 +285,7 @@
 				t.Errorf("windows is assumed to be disabled by default")
 			}
 			config.config.Targets[Windows] = []Target{
-				{Windows, Arch{ArchType: X86_64}, NativeBridgeDisabled, "", ""},
+				{Windows, Arch{ArchType: X86_64}, NativeBridgeDisabled, "", "", true},
 			}
 
 			ctx := NewTestArchContext()