[llvm] -fsanitize=function: fix .subsections_via_symbols (PR #87527)

[Fangrui Song via llvm-commits]

================
@@ -8,6 +9,15 @@
 ; CHECK-NEXT:   # %bb.0:
 ; CHECK-NEXT:   retq
 
+; MACHO:      ltmp0
----------------
MaskRay wrote:

`; MACHO:      ltmp0:`

add a colon to ensure we are testing a label

https://github.com/llvm/llvm-project/pull/87527