[Driver] Allow -fbasic-block-sections for AArch64 ELF (#80916)

Basic block sections "all" doesn't work on AArch64 since branch
relaxation may create new basic blocks. However, the other basic
block section modes should work out of the box since machine function
splitting already uses the basic block sections pass.

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5972,6 +5972,14 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
             << A->getAsString(Args) << A->getValue();
       else
         A->render(Args, CmdArgs);
+    } else if (Triple.isAArch64() && Triple.isOSBinFormatELF()) {
+      // "all" is not supported on AArch64 since branch relaxation creates new
+      // basic blocks for some cross-section branches.
+      if (Val != "labels" && Val != "none" && !Val.starts_with("list="))
+        D.Diag(diag::err_drv_invalid_value)
+            << A->getAsString(Args) << A->getValue();
+      else
+        A->render(Args, CmdArgs);
     } else if (Triple.isNVPTX()) {
       // Do not pass the option to the GPU compilation. We still want it enabled
       // for the host-side compilation, so seeing it here is not an error.

clang/test/Driver/fbasic-block-sections.c

@@ -2,6 +2,10 @@
 // RUN: %clang -### --target=x86_64 -fbasic-block-sections=all %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-ALL %s
 // RUN: %clang -### --target=x86_64 -fbasic-block-sections=list=%s %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-LIST %s
 // RUN: %clang -### --target=x86_64 -fbasic-block-sections=labels %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-LABELS %s
+// RUN: %clang -### --target=aarch64 -fbasic-block-sections=none %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-NONE %s
+// RUN: %clang -### --target=aarch64 -fbasic-block-sections=list=%s %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-LIST %s
+// RUN: %clang -### --target=aarch64 -fbasic-block-sections=labels %s -S 2>&1 | FileCheck -check-prefix=CHECK-OPT-LABELS %s
+// RUN: not %clang -### --target=aarch64 -fbasic-block-sections=all %s -S 2>&1 | FileCheck -check-prefix=CHECK-INVALID-VALUE %s
 // RUN: not %clang -c --target=arm-unknown-linux -fbasic-block-sections=all %s -S 2>&1 | FileCheck -check-prefix=CHECK-TRIPLE %s
 // RUN: %clang -### --target=arm-unknown-linux -fbasic-block-sections=all -fbasic-block-sections=none %s -S 2>&1 \
 // RUN:   | FileCheck -check-prefix=CHECK-NOOPT %s