[compiler-rt] [test] Handle missing ld.gold gracefully

Fix the is_binutils_lto_supported() function to handle missing
executables gracefully.  Currently, the function does not catch
exceptions from subprocess.Popen() and therefore causes lit to crash
if config.gold_executable does not specify a valid executable:

```
lit: /usr/lib/python3.11/site-packages/lit/TestingConfig.py:136: fatal: unable to parse config file '/tmp/portage/sys-libs/compiler-rt-
15.0.0/work/compiler-rt/test/lit.common.cfg.py', traceback: Traceback (most recent call last):
  File "/usr/lib/python3.11/site-packages/lit/TestingConfig.py", line 125, in load_from_path
    exec(compile(data, path, 'exec'), cfg_globals, None)
  File "/tmp/portage/sys-libs/compiler-rt-15.0.0/work/compiler-rt/test/lit.common.cfg.py", line 561, in <module>
    if is_binutils_lto_supported():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/portage/sys-libs/compiler-rt-15.0.0/work/compiler-rt/test/lit.common.cfg.py", line 543, in is_binutils_lto_supported
    ld_cmd = subprocess.Popen([exe, '--help'], stdout=subprocess.PIPE, env={'LANG': 'C'})
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 1022, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.11/subprocess.py", line 1899, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'GOLD_EXECUTABLE-NOTFOUND'
```

Differential Revision: https://reviews.llvm.org/D133358