[lld][test] Fix AArch64 build attribute test cleanup

This test has been reported to fail on a release bot, but nothing
in the content of the test on the release branch should cause this problem:
ld.lld: error: cannot open output file /home/buildbot/as-worker-92/clang-with-thin-lto-ubuntu-rel/build/stage1/tools/lld/test/ELF/Output/aarch64-build-attributes.s.tmp: Is a directory

The reason this is happening is that if the same builder happened to
run the extended version of this test, from a build of main, it would
create a directory with that name. The test on main stashes the
temporary files in that directory.

$ tree tools/lld/test/
tools/lld/test/
├── CMakeFiles
├── ELF
│   └── Output
│       ├── aarch64-build-attributes.s.tmp
│       │   ├── pauth-bti-gcs.s
│       │   └── pauth-bti-pac.s
│       ├── aarch64-build-attributes.s.tmp.merged.o
│       ├── aarch64-build-attributes.s.tmp1.o
│       ├── aarch64-build-attributes.s.tmp2.o
│       └── aarch64-build-attributes.s.tmp3.o
├── Unit
│   └── lit.site.cfg.py
├── cmake_install.cmake
└── lit.site.cfg.py

If you then had a 21.x build run, it would find that pre-existing
directory and try to write to it as if it were a file.

ld.lld: error: cannot open output file /home/david.spickett/build-llvm-aarch64/tools/lld/test/ELF/Output/aarch64-build-attributes.s.tmp: Is a directory

To fix this, remove the file or directory names we're going to use,
before the test starts.

(if this were main I'd put all the files in one direcory, but in the
interest of keeping release changes small I'm just adding the rm
line)

Author: DavidSpickett

lld/test/ELF/aarch64-build-attributes.s

@@ -1,4 +1,5 @@
 // REQUIRES: aarch64
+// RUN: rm -rf %t %t.o %t.so %t2.o
 // RUN: llvm-mc -triple=aarch64 %s -filetype=obj -o %t.o
 // RUN: ld.lld %t.o --shared -o %t.so
 // RUN: llvm-readelf --sections %t.so | FileCheck %s