OUTPUT_FORMAT is needed as the first line of libc.so (a linker script). The string comes from either

(a) ld --verbose on /dev/null, get the output (the OUTPUT_FORMAT line and the continuation lines), feed the output to scripts/output-format.sed
(b) ld --print-output-format

Neither choice makes sense for LLD. It supports multi targets and there is not a preferred default target. Like gold, it does not have the concept of "an internal linker script". ld.lld --verbose does not print OUTPUT_FORMAT.

As an alternative, we can get the string from objdump -f

```
% objdump -f /lib/x86_64-linux-gnu/libc.so.6 

/lib/x86_64-linux-gnu/libc.so.6:     file format elf64-x86-64

# LLVM>=11.0.0 (since https://reviews.llvm.org/D76046 )
% llvm-objdump -f /lib/x86_64-linux-gnu/libc.so.6

/lib/x86_64-linux-gnu/libc.so.6:        file format elf64-x86-64

architecture: x86_64
start address: 0x0000000000026e00
```