[fir] Force target for FIR to LLVM IR conversion

Author: clementval

flang/test/Fir/convert-to-llvm-invalid.fir

@@ -1,6 +1,6 @@
 // Test FIR to LLVM IR conversion invalid cases and diagnostics.
 
-// RUN: fir-opt --split-input-file --fir-to-llvm-ir --verify-diagnostics %s
+// RUN: fir-opt --split-input-file --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" --verify-diagnostics %s
 
 func @zero_aggregate() {
   // expected-error@+1{{failed to legalize operation 'fir.zero_bits'}}

flang/test/Fir/types-to-llvm.fir

@@ -1,6 +1,6 @@
 // Test FIR types conversion.
 
-// RUN: fir-opt --split-input-file --fir-to-llvm-ir %s | FileCheck %s
+// RUN: fir-opt --split-input-file --fir-to-llvm-ir="target=x86_64-unknown-linux-gnu" %s | FileCheck %s
 
 
 // Test sequence types `!fir.array`