This patch is missing from glibc 2.28 and is needed to support
/proc/self lookup when using the Hurd's procfs, which in turn is needed
for our 'guile-relocatable.patch'.
See <https://lists.gnu.org/archive/html/bug-hurd/2018-12/msg00024.html>.
* gnu/packages/patches/glibc-hurd-magic-pid.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/base.scm (glibc)[arguments]: Add 'apply-hurd-patch'
phase.
[native-inputs]: Add "hurd-magic-pid-patch" input.
* gnu/packages/cross-base.scm (cross-libc)[arguments]: Duplicate
'apply-hurd-patch' phase.
Ludovic Courtès