Fix template method InstARM32FourAddrFP to only have one definition.

Fixes case where IceTargetLowring.cpp and IceInstARM32.cpp generate
implementations for emitIAS().

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1638123002 .
diff --git a/src/IceInstARM32.h b/src/IceInstARM32.h
index eda1fe4..cca59d0 100644
--- a/src/IceInstARM32.h
+++ b/src/IceInstARM32.h
@@ -835,7 +835,7 @@
       return;
     emitFourAddrFP(Opcode, this, Func);
   }
-  void emitIAS(const Cfg *Func) const override { emitUsingTextFixup(Func); }
+  void emitIAS(const Cfg *Func) const override;
   void dump(const Cfg *Func) const override {
     if (!BuildDefs::dump())
       return;