linux-user: Define ELF_ET_DYN_BASE in $guest/target_mman.h

Copy each guest kernel's default value, then bound it
against reserved_va or the host address space.

Tested-by: Helge Deller <deller@gmx.de>
Reviewed-by: Helge Deller <deller@gmx.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index 84436d4..9aab48d 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -300,6 +300,7 @@
 }
 
 abi_ulong task_unmapped_base;
+abi_ulong elf_et_dyn_base;
 abi_ulong mmap_next_start;
 
 /*