[lld][test] Fix file cleanup in aarch64-build-attributes.s

This test made the foolish decision to take the lit documentation
at its word:
https://llvm.org/docs/CommandGuide/lit.html#substitutions

"%t temporary file name unique to the test"

%t is in fact the **path** of a file. As suggested by the line
below that describing %basename_t.

This test assumed it was just the filename itself and so left a
layout of:
$ 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

Note how the 2 .s files are in the temp dir but the .o files
are not.

To fix this, remove %t from all the temp file names so they
are created in the temp dir, which is cleaned before each run.

New layout:
$ tree tools/lld/test/
tools/lld/test/
├── CMakeFiles
├── ELF
│   └── Output
│       └── aarch64-build-attributes.s.tmp
│           ├── 1.o
│           ├── 2.o
│           ├── 3.o
│           ├── merged.o
│           ├── pauth-bti-gcs.s
│           └── pauth-bti-pac.s
├── Unit
│   └── lit.site.cfg.py
├── cmake_install.cmake
└── lit.site.cfg.py

Author: DavidSpickett

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

@@ -1,11 +1,11 @@
 // REQUIRES: aarch64
 // RUN: rm -rf %t && split-file %s %t && cd %t
 
-// RUN: llvm-mc -triple=aarch64 -filetype=obj %s -o %t1.o
-// RUN: llvm-mc -triple=aarch64 -filetype=obj pauth-bti-gcs.s -o %t2.o
-// RUN: llvm-mc -triple=aarch64 -filetype=obj pauth-bti-pac.s -o %t3.o
-// RUN: ld.lld -r %t1.o %t2.o %t3.o -o %t.merged.o
-// RUN: llvm-readelf -n %t.merged.o | FileCheck %s --check-prefix=NOTE
+// RUN: llvm-mc -triple=aarch64 -filetype=obj %s -o 1.o
+// RUN: llvm-mc -triple=aarch64 -filetype=obj pauth-bti-gcs.s -o 2.o
+// RUN: llvm-mc -triple=aarch64 -filetype=obj pauth-bti-pac.s -o 3.o
+// RUN: ld.lld -r 1.o 2.o 3.o -o merged.o
+// RUN: llvm-readelf -n merged.o | FileCheck %s --check-prefix=NOTE
 
 /// This test merges three object files with AArch64 build attributes.
 /// All contain identical PAuth ABI info (platform/version), which must be preserved.