[clang] [flang] [flang][nfc] Refactor linker invocation logic (PR #75534)

[Pete Steinfeld via cfe-commits]

https://github.com/psteinfeld commented:

After this change, I can no longer use flang-new to create an executable program.  When I try, I get the following output:
```
[psteinfeld at ice4 build]$ flang-new hello.f90 
/opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../bin/ld: /local/home/psteinfeld/up/build/lib/libFortran_main.a(Fortran_main.c.o): in function `main':
Fortran_main.c:(.text.startup.main+0xf): undefined reference to `_FortranAProgramStart'
/opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../bin/ld: Fortran_main.c:(.text.startup.main+0x19): undefined reference to `_FortranAProgramEndStatement'
flang-new: error: linker command failed with exit code 1 (use -v to see invocation)

```

https://github.com/llvm/llvm-project/pull/75534