Bug#1129746: glibc is not built with -fstack-clash-protection (despite it being in dpkg-buildflags)

Package: libc6
Version: 2.42-13

My understanding is that dpkg-buildflags defaults include
-fstack-clash-protection (see
https://git.dpkg.org/cgit/dpkg/dpkg.git/commit/?id=11efff1bf), but
somehow Debian's glibc appears to not be built with
-fstack-clash-protection; there are unprotected large stack
allocations in functions like ____strtold_l_internal,
____strtof128_l_internal, tempnam, tmpfile64, and more:

$ wget http://ftp.ch.debian.org/debian/pool/main/g/glibc/libc6_2.42-13_amd64.deb
[...]
$ ar x libc6_2.42-13_amd64.deb
$ tar xf data.tar.xz
$ objdump -d usr/lib/x86_64-linux-gnu/libc.so.6 | egrep 'sub
*\$0x.{4,},%rsp' | grep -v 0xffffff
   51763:       48 81 ec 48 36 00 00    sub    $0x3648,%rsp
   573c2:       48 81 ec 88 36 00 00    sub    $0x3688,%rsp
   611ca:       48 81 ec 10 10 00 00    sub    $0x1010,%rsp
   617a3:       48 81 ec 28 10 00 00    sub    $0x1028,%rsp
   a14f1:       48 81 ec b8 11 00 00    sub    $0x11b8,%rsp
   adeef:       48 81 ec 10 40 00 00    sub    $0x4010,%rsp
   adf11:       48 81 ec 10 10 00 00    sub    $0x1010,%rsp
   c8de0:       48 81 ec 58 36 00 00    sub    $0x3658,%rsp
   cb762:       48 81 ec 10 40 00 00    sub    $0x4010,%rsp
   cb775:       48 81 ec 10 10 00 00    sub    $0x1010,%rsp
   cde80:       48 81 ec 88 36 00 00    sub    $0x3688,%rsp
   e21ed:       48 81 ec d8 28 00 00    sub    $0x28d8,%rsp
   ea72d:       48 81 ec a8 15 00 00    sub    $0x15a8,%rsp
   f395e:       48 81 ec c8 48 00 00    sub    $0x48c8,%rsp
  106762:       48 81 ec 18 10 00 00    sub    $0x1018,%rsp
  10b160:       48 81 ec 18 10 00 00    sub    $0x1018,%rsp
  126a0b:       48 81 ec 00 10 00 00    sub    $0x1000,%rsp
  12d2ca:       48 81 ec 18 20 00 00    sub    $0x2018,%rsp
  14fb59:       48 81 ec c8 29 00 00    sub    $0x29c8,%rsp
  150d76:       48 81 ec 88 22 00 00    sub    $0x2288,%rsp
  161fee:       48 81 ec c0 11 00 00    sub    $0x11c0,%rsp
  162530:       48 81 ec 48 10 00 00    sub    $0x1048,%rsp

Since glibc does not have stack clash protection, the stack clash
protection of any software that calls glibc functions with large stack
frames is also less effective.

Can you please also build glibc with -fstack-clash-protection?