[Thumb] Baseline test for incorrect relocation with -ffunction-sections

When calling a dso_local function, we end up creating a call against the
.Lfoo$local label. This might be converted to a relocation against a
section if there is such a matching one (which is a lot more likely with
-ffunction-sections) and then the LSB (Thumb flag) will be lost.

I originally noticed this with Morello LLVM (which uses the LSB to indicate
a C64 encoding mode function). The missing LSB meant that ld.lld would
insert a thunk that switches encoding mode which then resulted in errors
at runtime since functions were being entered with the wrong encoding mode.
Since the Morello backend is not upstream, I looked if any in-tree
backends could also be affected by the missing STT_FUNC flag and noticed
that Thumb is also affected (although the bug is rather difficult to
trigger - it currently requires inline assembly).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D131432