sim: Add gtoh/htog helpers that take an explicit endianness

sim: Add gtoh/htog helpers that take an explicit endianness

Add helper functions to swap between guest byte order and host byte
order that take a guest endianness as a parameter. These functions are
called htog and htog to be consistent with the helper functions that
extract guest byte order from a compile time constant.

Change-Id: Ie6be7dfd3b7a58ad6bfb57b25be5f85b5f425929
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8201
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>