glibc: Pull latest 2.31 HEAD

The relevant commit log:

$ git log --format="%h %s" df31c7ca927242d5d4eee97f93a01e23ff47e332..f84949f1c4bbf20e6a1d9a5859cf012cde060ede
f84949f1c4 powerpc64: Workaround sigtramp vdso return call
5e43566f0f nscd: Fix double free in netgroupcache [BZ #27462]
d0c84d22b6 gconv: Fix assertion failure in ISO-2022-JP-3 module (bug 27256)
af316e4627 x86: Check IFUNC definition in unrelocated executable [BZ #20019]
36eb01dd85 x86: Set header.feature_1 in TCB for always-on CET [BZ #27177]
8b7be87aa2 x86-64: Avoid rep movsb with short distance [BZ #27130]
c4f5e32aae Fix buffer overrun in EUC-KR conversion module (bz #24973)
0858f46440 Add NEWS entry for CVE-2020-29562 (BZ #26923)
1e40391de2 iconv: Fix incorrect UCS4 inner loop bounds (BZ#26923)
568c86274a tests-mcheck: New variable to run tests with MALLOC_CHECK_=3

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>