[Driver] Add -mbranch-protection to ARM and AArch64 multilib flags (#106391)

This adds the `-mbranch-protection` command line option to the set of
flags used by the multilib selection for ARM and AArch64 targets.

Author: pratlucas

clang/lib/Driver/ToolChain.cpp

@@ -221,6 +221,12 @@ static void getAArch64MultilibFlags(const Driver &D,
   assert(!ArchName.empty() && "at least one architecture should be found");
   MArch.insert(MArch.begin(), ("-march=" + ArchName).str());
   Result.push_back(llvm::join(MArch, "+"));
+
+  const Arg *BranchProtectionArg =
+      Args.getLastArgNoClaim(options::OPT_mbranch_protection_EQ);
+  if (BranchProtectionArg) {
+    Result.push_back(BranchProtectionArg->getAsString(Args));
+  }
 }
 
 static void getARMMultilibFlags(const Driver &D,
@@ -268,6 +274,12 @@ static void getARMMultilibFlags(const Driver &D,
   case arm::FloatABI::Invalid:
     llvm_unreachable("Invalid float ABI");
   }
+
+  const Arg *BranchProtectionArg =
+      Args.getLastArgNoClaim(options::OPT_mbranch_protection_EQ);
+  if (BranchProtectionArg) {
+    Result.push_back(BranchProtectionArg->getAsString(Args));
+  }
 }
 
 static void getRISCVMultilibFlags(const Driver &D, const llvm::Triple &Triple,

clang/test/Driver/print-multi-selection-flags.c

@@ -59,6 +59,10 @@
 // CHECK-SVE2: --target=aarch64-unknown-none-elf
 // CHECK-SVE2: -march=armv{{.*}}-a{{.*}}+simd{{.*}}+sve{{.*}}+sve2{{.*}}
 
+// RUN: %clang -print-multi-flags-experimental --target=arm-none-eabi -mbranch-protection=standard    | FileCheck --check-prefix=CHECK-BRANCH-PROTECTION %s
+// RUN: %clang -print-multi-flags-experimental --target=aarch64-none-elf -mbranch-protection=standard | FileCheck --check-prefix=CHECK-BRANCH-PROTECTION %s
+// CHECK-BRANCH-PROTECTION: -mbranch-protection=standard
+
 // RUN: %clang -print-multi-flags-experimental --target=riscv32-none-elf -march=rv32g | FileCheck --check-prefix=CHECK-RV32 %s
 // CHECK-RV32: --target=riscv32-unknown-none-elf
 // CHECK-RV32: -mabi=ilp32d