This patch is taken from debian's glibc package (generate-supported.mk).
It install the localedata/SUPPORTED file of the glibc. This file lists
all the supported locales of the glibc.

diff --git a/localedata/Makefile b/localedata/Makefile
--- a/localedata/Makefile
+++ b/localedata/Makefile
@@ -176,7 +176,8 @@
 else
 install-others := $(addprefix $(inst_i18ndir)/, \
 			      $(addsuffix .gz, $(charmaps)) \
-			      $(locales))
+			      $(locales)) \
+                   $(inst_i18ndir)/SUPPORTED
 endif
 
 tests: $(objdir)/iconvdata/gconv-modules
@@ -401,6 +402,14 @@
 
 include SUPPORTED
 
+$(inst_i18ndir)/SUPPORTED: SUPPORTED $(+force)
+	for locale in $(SUPPORTED-LOCALES); do \
+		[ $$locale = true ] && continue; \
+		echo $$locale | sed 's,/, ,' >> LOCALES; \
+	done
+	$(make-target-directory)
+	$(INSTALL_DATA) LOCALES $@
+
 INSTALL-SUPPORTED-LOCALE-ARCHIVE=$(addprefix install-archive-, $(SUPPORTED-LOCALES))
 INSTALL-SUPPORTED-LOCALE-FILES=$(addprefix install-files-, $(SUPPORTED-LOCALES))
 
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<div class='path'>path: <a href='/guix/log/?id=9090f7bf6999d56d9bc74669b2c839fd5739a250'>root</a>/<a href='/guix/log/gnu?id=9090f7bf6999d56d9bc74669b2c839fd5739a250'>gnu</a>/<a href='/guix/log/gnu/tests.scm?id=9090f7bf6999d56d9bc74669b2c839fd5739a250'>tests.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/tests.scm?id=9090f7bf6999d56d9bc74669b2c839fd5739a250&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-25 19:11:36 +0200'>2020-07-25</span></td><td><a href='/guix/commit/gnu/tests.scm?id=a5e2fc73760a2ae023f2e56bdbf8025971f90e64'>utils: Move &lt;location&gt; and '&amp;error-location' to (guix diagnostics).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/utils.scm (&lt;location&gt;, source-properties-&gt;location)
(location-&gt;source-properties, &amp;error-location): Move to...
* guix/diagnostics.scm: ... here.
* gnu.scm: Adjust imports accordingly.
* gnu/machine.scm: Likewise.
* gnu/system.scm: Likewise.
* gnu/tests.scm: Likewise.
* guix/inferior.scm: Likewise.
* tests/channels.scm: Likewise.
* tests/packages.scm: Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>