[llvm] [Bolt] fix a wrong relocation update issue with weak references (PR #69136)

[Selin Dag via llvm-commits]

s-dag wrote:

Hi,

Wanted to report this seems to be the same issue we ran into while trying to apply BOLT to an Aarch64 binary dynamically linked with musl. So it is a very important use case for embedded software world and we expect more people will run into it as BOLT Aarch64/embedded usage picks up. It can be reproduced with a simple hello_world. Reproduction steps are below:
```
cat hello.c
#include <stdio.h>

int main() {
     printf("Hello World");
     return 0;
}
```

1- `aarch64-unknown-linux-musl-gcc hello.c -Wl,-dynamic-linker=ld-musl-aarch64.so.1 -Wl,-emit-relocs -o hello_aarch64`

2- `llvm-bolt hello_aarch64 -o hello_bolted_skip --skip-funcs=.*  --use-gnu-stack`

`hello_bolted_skip` segfaults at start. When we compared disassembly, we saw .rodata had differences which boiled down  to `<__register_frame_info at plt>` /`<__deregister_frame_info at plt>` relocs added by BOLT that are actually undefined weak references and are not defined in musl. 

The patch in https://github.com/llvm/llvm-project/pull/69136/commits/2acdf00a1882efe43b3efec778b2d68600bfb807 does fix this reproducer as well as our own binary. We hope to see it merged soon.

https://github.com/llvm/llvm-project/pull/69136