Provide a 'mach_print' symbol in libc.

--- a/sysdeps/mach/hurd/i386/libc.abilist	2020-04-01 19:22:44.710088821 +0200
+++ b/sysdeps/mach/hurd/i386/libc.abilist	2020-04-01 19:22:52.062051161 +0200
@@ -338,6 +338,7 @@
 GLIBC_2.2.6 __mach_port_allocate F
 GLIBC_2.2.6 __mach_port_deallocate F
 GLIBC_2.2.6 __mach_port_insert_right F
+GLIBC_2.21 __mach_print F
 GLIBC_2.2.6 __mach_reply_port F
 GLIBC_2.2.6 __mach_task_self_ D 0x4
 GLIBC_2.2.6 __mach_thread_self F

--- a/mach/Versions
+++ b/mach/Versions
@@ -56,6 +56,7 @@
   }
   GLIBC_2.21 {
     __mach_host_self_;
+    mach_print;
   }
 
   HURD_CTHREADS_0.3 {
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<tr><td><span title='2017-05-04 22:45:53 +0300'>2017-05-04</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=97658183eda6e196bd16bed02838d321b45aee70'>etc: indent-code.el: Define source tree by "indent-code.el".</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>
<tr><td><span title='2017-01-20 21:39:17 +0100'>2017-01-20</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=830d4c1fcdff7be199e66cd70a5048264b54a151'>etc: indent-code.el: Add indentation info for non-Guix Scheme constructs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-13 18:49:31 +0100'>2017-01-13</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=557d9c8d7a843bf06e75b4e1a742654ccf951fa3'>etc: Support indentation of whole files.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>