linux-user: fix mips 32-on-64 prealloc case

MIPS only supports 31 bits of virtual address space for user space, so let's
make sure we stay within that limit with our preallocated memory block.

This fixes the MIPS user space targets when executed without command line
option.
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

linux-user/main.c

@@ -57,7 +57,12 @@ int have_guest_base;
  * This way we will never overlap with our own libraries or binaries or stack
  * or anything else that QEMU maps.
  */
+# ifdef TARGET_MIPS
+/* MIPS only supports 31 bits of virtual address space for user space */
+unsigned long reserved_va = 0x77000000;
+# else
 unsigned long reserved_va = 0xf7000000;
+# endif
 #else
 unsigned long reserved_va;
 #endif