Resolve or silence some warnings (#1905)

* Resolve some warnings

- Their frequent appearance in the build logs is driving me nuts

* Silence warnings about `offsetof`

* Don't apply `-Wno-invalid-offset` to C, only to C++

src/ARMJIT_A64/ARMJIT_Compiler.h

@@ -69,7 +69,7 @@ struct Op2
     bool IsSimpleReg()
     { return !IsImm && !Reg.ShiftAmount && Reg.ShiftType == Arm64Gen::ST_LSL; }
     bool ImmFits12Bit()
-    { return IsImm && (Imm & 0xFFF == Imm); }
+    { return IsImm && ((Imm & 0xFFF) == Imm); }
     bool IsZero()
     { return IsImm && !Imm; }