[flang] Fix triple in a couple of driver tests

In https://reviews.llvm.org/D124667, I added tests that check the
generated assembly. I verified the assembly on AArch64 and X86_64, but
the PPC Flang buildbot [1] started failing (i.e. the assembly was not
generic enough).

In order to fix this, I'm changing these tests to be only run on
AAarch64 - that's the architecture that most of public Flang buildbots
use.

I'm hoping that this is straightforward enough and am merging it without
a review.

[1] https://lab.llvm.org/buildbot/#/builders/21/builds/40256

flang/test/Driver/emit-asm-from-llvm-bc.ll

@@ -1,19 +1,18 @@
-; Verify that the driver can consume LLVM BC files. The expected assembly is
-; fairly generic (tested on AArch64 and X86_64), but we may need to tweak when
-; testing on other platforms. Note that the actual output doesn't matter as
-; long as it's in Assembly format.
+; Verify that the driver can consume LLVM BC files.
+
+; REQUIRES: aarch64-registered-target
 
 ;-------------
 ; RUN COMMANDS
 ;-------------
 ; RUN: rm -f %t.bc
-; RUN: %flang_fc1 -emit-llvm-bc %s -o %t.bc
-; RUN: %flang_fc1 -S -o - %t.bc | FileCheck %s
+; RUN: %flang_fc1 -triple aarch64-unknown-linux-gnu -emit-llvm-bc %s -o %t.bc
+; RUN: %flang_fc1 -S -triple aarch64-unknown-linux-gnu -o - %t.bc | FileCheck %s
 ; RUN: rm -f %t.bc
 
 ; RUN: rm -f %t.bc
-; RUN: %flang -c -emit-llvm %s -o %t.bc
-; RUN: %flang -S -o - %t.bc | FileCheck %s
+; RUN: %flang -c -target aarch64-unknown-linux-gnu -emit-llvm %s -o %t.bc
+; RUN: %flang -S -target aarch64-unknown-linux-gnu -o - %t.bc | FileCheck %s
 ; RUN: rm -f %t.bc
 
 ;----------------

flang/test/Driver/emit-asm-from-llvm.ll

@@ -1,13 +1,12 @@
-; Verify that the driver can consume LLVM IR files. The expected assembly is
-; fairly generic (verified on AArch64 and X86_64), but we may need to tweak when
-; testing on other platforms. Note that the actual output doesn't matter
-; as long as it's in Assembly format.
+; Verify that the driver can consume LLVM IR files.
+
+; REQUIRES: aarch64-registered-target
 
 ;-------------
 ; RUN COMMANDS
 ;-------------
-; RUN: %flang_fc1 -S %s -o - | FileCheck %s
-; RUN: %flang -S  %s -o - | FileCheck %s
+; RUN: %flang_fc1 -S -triple aarch64-unknown-linux-gnu %s -o - | FileCheck %s
+; RUN: %flang -S -target aarch64-unknown-linux-gnu %s -o - | FileCheck %s
 
 ;----------------
 ; EXPECTED OUTPUT