[BPF] Remove unused weak symbol __bpf_trap (#166003)
Nikita Popov reported an issue ([1]) where a dangling weak symbol
__bpf_trap is in the final binary and this caused libbpf failing like
below:
$ veristat -v ./t.o
Processing 't.o'...
libbpf: elf: skipping unrecognized data section(4) .eh_frame
libbpf: elf: skipping relo section(5) .rel.eh_frame for section(4) .eh_frame
libbpf: failed to find BTF for extern '__bpf_trap': -3
Failed to open './t.o': -3
In llvm, the dag selection phase generates __bpf_trap in code. Later the
UnreachableBlockElim pass removed __bpf_trap from the code, but
__bpf_trap symbol survives in the symbol table.
Having a dangling __bpf_trap weak symbol is not good for old kernels as
seen in the above veristat failure. Although users could use compiler
flag `-mllvm -bpf-disable-trap-unreachable` to workaround the issue,
this patch fixed the issue by removing the dangling __bpf_trap.
[1] https://github.com/llvm/llvm-project/issues/165696
