Initial Fuchsia support.

This change adds Fuchsia as a valid OS. Future changes
will add proper toolchain support.

Bug: 119831161
Test: Compile walleye. Confirm that adding `fuchsia` to the
os-specific declaration of an arbitrary target does not throw
a build error.
Change-Id: I64eb928afb7512f3fbe32abb313b4c3efe16b169

android/module.go

@@ -64,6 +64,7 @@ type androidBaseContext interface {
 	Host() bool
 	Device() bool
 	Darwin() bool
+	Fuchsia() bool
 	Windows() bool
 	Debug() bool
 	PrimaryArch() bool
@@ -1121,6 +1122,10 @@ func (a *androidBaseContextImpl) Darwin() bool {
 	return a.target.Os == Darwin
 }
 
+func (a *androidBaseContextImpl) Fuchsia() bool {
+	return a.target.Os == Fuchsia
+}
+
 func (a *androidBaseContextImpl) Windows() bool {
 	return a.target.Os == Windows
 }