bin/symbols-check: Fix undefined symbol detection on macOS

Commit e626636e90 ("bin/symbols-check: fix fields length condition
before accessing fields") changed the condition from `or` to `and`
to prevent potential IndexError when accessing fields[1].

However, this broke macOS because `nm -gP` outputs different field
counts for undefined symbols:
- Linux:  `_symbol U`           (2 fields)
- macOS:  `_symbol U 0 0`       (4 fields)

The condition `len(fields) == 2 and fields[1] == 'U'` only matched
the Linux format, causing undefined symbols like
_mesa_glapi_tls_Dispatch to be incorrectly reported as
"unknown symbol exported" on macOS.

Fix by using `len(fields) >= 2` to handle both platforms while still
guarding against IndexError.

Fixes: e626636e90 ("bin/symbols-check: fix fields length condition before accessing fields")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/13451
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
(cherry picked from commit 191e4b8fe0)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39117>

.pick_status.json

@@ -974,7 +974,7 @@
         "description": "bin/symbols-check: Fix undefined symbol detection on macOS",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "e626636e90c39e897ccd99d5ef77f5b9f94e7989",
         "notes": null

bin/symbols-check.py

@@ -80,7 +80,7 @@ def get_symbols_nm(nm, lib):
         if line.startswith(' '):
             continue
         fields = line.split()
-        if len(fields) == 2 and fields[1] == 'U':
+        if len(fields) >= 2 and fields[1] == 'U':
             continue
         symbol_name = fields[0]
         if platform_name == 'Linux' or platform_name == 'GNU' or platform_name.startswith('GNU/'):