doc: Mark up __cxa_atexit as @code.

gcc:
	* doc/install.texi (Configuration): Mark up __cxa_atexit as @code.

gcc/doc/install.texi

@@ -1779,12 +1779,12 @@ Produce code conforming to version 20191213.
 In the absence of this configuration option the default version is 20191213.
 
 @item --enable-__cxa_atexit
-Define if you want to use __cxa_atexit, rather than atexit, to
+Define if you want to use @code{__cxa_atexit}, rather than atexit, to
 register C++ destructors for local statics and global objects.
 This is essential for fully standards-compliant handling of
-destructors, but requires __cxa_atexit in libc.  This option is currently
-only available on systems with GNU libc.  When enabled, this will cause
-@option{-fuse-cxa-atexit} to be passed by default.
+destructors, but requires @code{__cxa_atexit} in libc.  This option is
+currently only available on systems with GNU libc.  When enabled, this
+will cause @option{-fuse-cxa-atexit} to be passed by default.
 
 @item --enable-gnu-indirect-function
 Define if you want to enable the @code{ifunc} attribute.  This option is