Fix memory access from signed 32bit inferior registers on 64bit GDB.
* linux-nat.c (linux_xfer_partial <TARGET_OBJECT_MEMORY>): New variable
addr_bit. Mask OFFSET by the ADDR_BIT width.
gdb/testsuite/
* gdb.arch/amd64-i386-address.exp, gdb.arch/amd64-i386-address.S: New.