[gdb/contrib] Drop two words from codespell-ignore-words.txt

Tom Tromey mentioned [1] that the words "invokable" and "useable"
present in codespell-ignore-words.txt should be dropped.

Do so and fix the following typos:
...
$ codespell --config gdbsupport/setup.cfg gdbsupport
gdbsupport/common-debug.h:218: invokable ==> invocable
gdbsupport/event-loop.cc:84: useable ==> usable
...

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://sourceware.org/pipermail/gdb-patches/2025-March/216584.html

gdb/contrib/codespell-ignore-words.txt

diff --git a/gdb/contrib/codespell-ignore-words.txt b/gdb/contrib/codespell-ignore-words.txt
index 2b9d878..2d6e13a 100644
--- a/gdb/contrib/codespell-ignore-words.txt
+++ b/gdb/contrib/codespell-ignore-words.txt
@@ -1,4 +1,2 @@
 configury
-invokable
-useable
 SME

gdbsupport/common-debug.h

diff --git a/gdbsupport/common-debug.h b/gdbsupport/common-debug.h
index f7c8d81..547ea47 100644
--- a/gdbsupport/common-debug.h
+++ b/gdbsupport/common-debug.h
@@ -215,7 +215,7 @@ struct scoped_debug_start_end
   bool m_disabled = false;
 };
 
-/* Implementation of is_debug_enabled when PT is an invokable type.  */
+/* Implementation of is_debug_enabled when PT is an invocable type.  */
 
 template<typename PT>
 inline bool

gdbsupport/event-loop.cc

diff --git a/gdbsupport/event-loop.cc b/gdbsupport/event-loop.cc
index 4f7ad36..64cdb1f 100644
--- a/gdbsupport/event-loop.cc
+++ b/gdbsupport/event-loop.cc
@@ -81,7 +81,7 @@
 
 #ifdef HAVE_POLL
 /* Do we use poll or select?  Some systems have poll, but then it's
-   not useable with all kinds of files.  We probe that whenever a new
+   not usable with all kinds of files.  We probe that whenever a new
    file handler is added.  */
 static bool use_poll = true;
 #endif