Get rid of $(combo_target)

The only OS-ARCH combo that would have benefited from it
is linux-x86, but it explicitly used separate configurations
for the HOST_ and TARGET_ side of things.

This makes is clear which files are related to the HOST_
configuration and which ones are related to the TARGET_
configuration, and expands $(combo_target) to the only
possible/reasonable value that it could have had in every
file.

This also cleans up the simulator, by moving it in a single
place in TARGET_linux_x86 (since the only part that's special
is to use HOST_ settings even when building TARGET_ modules).

Change-Id: I2420eeb8cfe539f5807ec99cb3177ffb9f2476d5