[flang][test] Fix mtune test on AArch64 bots

The native architecture is AArch64 here so the pentium name won't
work even if you've got the x86 backend enabled.

https://lab.llvm.org/buildbot/#/builders/17/builds/898

Pass an explicit target for each run line to fix this.

Test added in f1d3fe7 / #98517

Author: DavidSpickett

flang/test/Lower/tune-cpu-llvm.f90

@@ -1,5 +1,5 @@
-! RUN: %if x86-registered-target %{ %flang -mtune=pentium4 -S -emit-llvm %s -o - | FileCheck %s --check-prefixes=ALL,CHECK-X86 %}
-! RUN: %if aarch64-registered-target %{ %flang -mtune=neoverse-n1 -S -emit-llvm %s -o - | FileCheck %s --check-prefixes=ALL,CHECK-ARM %}
+! RUN: %if x86-registered-target %{ %flang -target x86_64-linux-gnu -mtune=pentium4 -S -emit-llvm %s -o - | FileCheck %s --check-prefixes=ALL,CHECK-X86 %}
+! RUN: %if aarch64-registered-target %{ %flang -target aarch64-linux-gnu -mtune=neoverse-n1 -S -emit-llvm %s -o - | FileCheck %s --check-prefixes=ALL,CHECK-ARM %}
 
 !ALL: attributes #{{[0-9]+}} = {
 !CHECK-X86-SAME: "tune-cpu"="pentium4"