[AArch64] [test] Fix stack allocation instructions in the seh.s test

The actual unwind opcodes only stores stack increments in units
of 16 (which is what it is listed as in the unwind opcode
dumping by llvm-readobj); actually write what we intend to encode.

Author: mstorsjo

llvm/test/MC/AArch64/seh.s

@@ -139,8 +139,8 @@
     .endef
     .seh_proc func
 func:
-    sub sp, sp, #24
-    .seh_stackalloc 24
+    sub sp, sp, #16
+    .seh_stackalloc 16
     mov x29, sp
     .seh_set_fp
     stp x29, x30, [sp, #-32]!
@@ -210,8 +210,8 @@ func:
     .seh_endprologue
     nop
     .seh_startepilogue
-    add sp, sp, #24
-    .seh_stackalloc 24
+    add sp, sp, #16
+    .seh_stackalloc 16
     .seh_endepilogue
     ret
     .seh_handler __C_specific_handler, @except