Platform qemu-user: Build path to qemu automatically if not specified

Get the path to qemu in the following order:
1) From the property platform.plugin.qemu-user.emulator-path
2) If that property is not set, from PATH, building the name of the qemu
   binary from the triple in  property platform.plugin.qemu-user.architecture
3) If that property is not set, from PATH, building the name of the qemu
   binary from the triple in the target

This will allow a user to load a target and run without setting properties,
if qemu is on the PATH and named qemu-<ArchName>

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D155117