Repaired the garbled description at the top of check_clang.sh (#598)

The comment block describing what the script does was left half-rewritten when
the example link stage was added to it: one sentence ends mid-clause and the
word "profile" appears twice, once as the tail of a sentence that was meant to
be replaced.

Say the three stages plainly instead, and keep the point the truncated sentence
was making, that only the linking stage needs a target C library. This block is
what --help prints, so the damage was user visible.

Comment only; no behaviour change. Verified that --help still frames the
intended lines, since it selects them by number.

Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>

scripts/check_clang.sh

@@ -14,10 +14,10 @@
 # SPDX-License-Identifier: MIT and CC0-1.0
 ##############################################################################
 
-# Builds the Arm ports with an LLVM based toolchain: every assembly source of
-# every Arm gnu port, and the common C sources for one core per architecture
-# profile. Compilation only, no linking, so no target C library is required
-# profile, then links the example builds that have a script driver.
+# Builds the Arm ports with an LLVM based toolchain, in three stages: assemble
+# every assembly source of every Arm gnu port, compile the common C sources for
+# one core per architecture profile, then link the example builds that have a
+# script driver. Only that last stage needs a target C library.
 #
 #     scripts/check_clang.sh                       # clang from PATH
 #     scripts/check_clang.sh --clang /path/to/clang