[AArch64] [test] Write the seh.s test output object to a file

This is what is done in other tests; this makes it easier to
inspect the output of this test manually.

Author: mstorsjo

llvm/test/MC/AArch64/seh.s

@@ -1,6 +1,7 @@
 // This test checks that the SEH directives emit the correct unwind data.
 
-// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s | llvm-readobj -S -r -u - | FileCheck %s
+// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s -o %t.o
+// RUN: llvm-readobj -S -r -u %t.o | FileCheck %s
 
 // Check that the output assembler directives also can be parsed, and
 // that they produce equivalent output: