Make target_phys_addr_t 64 bits unconditionally

The hassle and compile time overhead of maintaining both 32-bit and 64-bit
capable source isn't worth the tiny performance advantage which is seen on
a minority of configurations.  Switch to compiling libhw only once, with
target_phys_addr_t unconditionally typedefed to uint64_t.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>