Support AT_USRSTACKBASE and AT_USRSTACKLIM.

FreeBSD's kernel has recently added two new ELF auxiliary vector
entries to describe the location of the user stack for the initial
thread in a process.

This change displays the proper name and description of these entries
in 'info auxv'.

gdb/fbsd-tdep.c

@@ -1571,6 +1571,8 @@ fbsd_print_auxv_entry (struct gdbarch *gdbarch, struct ui_file *file,
       TAG (PS_STRINGS, _("Pointer to ps_strings"), AUXV_FORMAT_HEX);
       TAG (FXRNG, _("Pointer to root RNG seed version"), AUXV_FORMAT_HEX);
       TAG (KPRELOAD, _("Base address of vDSO"), AUXV_FORMAT_HEX);
+      TAG (USRSTACKBASE, _("Top of user stack"), AUXV_FORMAT_HEX);
+      TAG (USRSTACKLIM, _("Grow limit of user stack"), AUXV_FORMAT_HEX);
     }
 
   fprint_auxv_entry (file, name, description, format, type, val);

include/elf/common.h

@@ -1405,6 +1405,8 @@
 #define AT_FREEBSD_PS_STRINGS   32      /* struct ps_strings. */
 #define AT_FREEBSD_FXRNG        33      /* Pointer to root RNG seed version. */
 #define AT_FREEBSD_KPRELOAD     34      /* Base of vdso. */
+#define AT_FREEBSD_USRSTACKBASE 35      /* Top of user stack. */
+#define AT_FREEBSD_USRSTACKLIM  36      /* Grow limit of user stack. */
 
 #define AT_SUN_UID      2000    /* Effective user ID.  */
 #define AT_SUN_RUID     2001    /* Real user ID.  */