[llvm] [BOLT][AArch64] Provide createDummyReturnFunction (PR #96626)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

### Force-pushed history:

Adding `%cflags` interfered with my previous c test `dummy-return.test`. I preferred not to expand that test (ie adding a `_start` method and letting the linker know about the entry method), but instead I've written a minimal assembly test.

I force-pushed, as I wanted to introduce the asm test on the initial 'showcase-commit'.


https://github.com/llvm/llvm-project/pull/96626