sim: switch sim_{read,write} APIs to 64-bit all the time [PR sim/7504]

sim: switch sim_{read,write} APIs to 64-bit all the time [PR sim/7504]

We've been using SIM_ADDR which has always been 32-bit.  This means
the upper 32-bit address range in 64-bit sims is inaccessible.  Use
64-bit addresses all the time since we want the APIs to be stable
regardless of the active arch backend (which can be 32 or 64-bit).

The length is also 64-bit because it's completely feasible to have
a program that is larger than 4 GiB in size/image/runtime.  Forcing
the caller to manually chunk those accesses up into 4 GiB at a time
doesn't seem useful to anyone.

Bug: https://sourceware.org/PR7504