[AArch64] Null check TargetStreamer before emitting .variant_pcs

This was causing crashes on Mach-O targets as we don't construct a
TargetStreamer for that object format. Other uses of the TargetStreamer
(TS) appear to be limited to ELF and COFF platforms (where it is none
null, so don't need changing).

Author: MacDue

llvm/test/CodeGen/AArch64/variant-pcs.ll

@@ -2,6 +2,9 @@
 ; RUN: llc -mtriple=aarch64-linux-gnu -mattr=+sve -filetype=obj -o - %s \
 ; RUN:   | llvm-readobj --symbols - | FileCheck %s --check-prefix=CHECK-OBJ
 
+; Check we don't crash when using a Mach-O object format.
+; RUN: llc -mtriple=arm64-apple-macosx15.0.0 -mattr=+sve -filetype=obj -o /dev/null %s
+
 define i32 @base_pcs() {
 ; CHECK-ASM-LABEL: base_pcs:
 ; CHECK-ASM-NOT: .variant_pcs