linkers: try a bit harder to autodetect the correct linker id

mingw GCC using ld.bfd emits diagnostics that include
"-plugin-opt=-pass-through=-lmoldname" and this triggers a match for
mold when it should not.

Instead, always check the very beginning of the output for the linker
id. This is pretty consistent:

- it is always on stdout

- GCC may put additional things on stderr we don't care about

- swift is bizarre and on some OSes redirects the linker stdout to
  swiftc's stderr, but it will still be the only stderr; we didn't even
  check stderr at all until commit 712cbe0568

For gold/bfd linkers, the linker id is always the *second* word, after
the legally mandated "GNU" we already check for.

mesonbuild/linkers/detect.py

@@ -194,9 +194,11 @@ def guess_nix_linker(env: 'Environment', compiler: T.List[str], comp_class: T.Ty
         linker = AppleDynamicLinker(compiler, for_machine, comp_class.LINKER_PREFIX, override, version=v)
     elif 'GNU' in o or 'GNU' in e:
         cls: T.Type[GnuDynamicLinker]
-        if 'gold' in o or 'gold' in e:
+        # this is always the only thing on stdout, except for swift
+        # which may or may not redirect the linker stdout to stderr
+        if o.startswith('GNU gold') or e.startswith('GNU gold'):
             cls = GnuGoldDynamicLinker
-        elif 'mold' in o or 'mold' in e:
+        elif o.startswith('mold') or e.startswith('mold'):
             cls = MoldDynamicLinker
         else:
             cls = GnuBFDDynamicLinker