gnu: perl: Actually produce a host perl when cross-compiling.

* gnu/packages/patches/perl-cross.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/perl.scm (perl)[native-inputs]: When cross-compiling, add
`perl-cross' input and perl-cross.patch.
[arguments]: When cross-compiling, use them to produce binary for host.

gnu/packages/patches/perl-cross.patch

@@ -0,0 +1,60 @@
+Submitted upstream.
+
+From 4c3eb19a11dfe5c88c902481a8294c4f675fcd03 Mon Sep 17 00:00:00 2001
+From: "Jan (janneke) Nieuwenhuizen" <janneke@gnu.org>
+Date: Sun, 19 Apr 2020 23:33:28 +0200
+Subject: [PATCH] Support cross-compiling to the Hurd.
+
+* cnf/hints/gnu: New file.
+* cnf/configure_tool.sh: Guess it.
+---
+ cnf/configure_tool.sh |  4 ++++
+ cnf/hints/gnu         | 21 +++++++++++++++++++++
+ 2 files changed, 25 insertions(+)
+ create mode 100644 cnf/hints/gnu
+
+diff --git a/cnf/configure_tool.sh b/cnf/configure_tool.sh
+index 32201c0..7b0baa4 100644
+--- a/cnf/configure_tool.sh
++++ b/cnf/configure_tool.sh
+@@ -266,6 +266,10 @@ if not hinted 'osname'; then
+ 			define osname "bsd"
+ 			result "BSD"
+ 			;;
++		*-gnu*)
++			define osname "gnu"
++			result "GNU"
++			;;
+ 		*)
+ 			result "no"
+ 			;;
+diff --git a/cnf/hints/gnu b/cnf/hints/gnu
+new file mode 100644
+index 0000000..a0583a2
+--- /dev/null
++++ b/cnf/hints/gnu
+@@ -0,0 +1,21 @@
++# Hurd syscalls
++d_voidsig='define'
++d_nanosleep='undef'
++d_clock_gettime='define'
++d_clock_getres='define'
++d_clock_nanosleep='define'
++d_clock='define'
++
++# From the original linux.sh
++usemallocwrap='define'
++
++# libraries to test
++libswanted='m crypt pthread nm ndbm gdbm dbm db dl gdbm_compat'
++
++d_procselfexe='undef'
++procselfexe='"undef"'
++
++st_ino_sign=1
++st_ino_size=8
++
++d_fcntl_can_lock='define'
+-- 
+2.26.0
+