Rearrange gdb/configure.nat to make it simpler and less redundant

The previous commit introduced gdb/configure.nat, but it was just a
copy-and-past (with the necessary adjustments) from the files under
gdb/config/.  We can do better than that.

Instead of using one big 'case' statement that matches the
${gdb_host_cpu} and then match each ${gdb_host}, it is possible to
remove a lof of redundancy by matching the most common ${gdb_host}'s
first, setting the common variables for each, and then proceed to
matching specific ${gdb_host}'s and ${gdb_host_cpu}'s.  In other
words, reverse the order of the 'case's and take advantage of the fact
that a lot of parameters are the same for each host.

This commit was tested on x86_64 without regressions.

gdb/ChangeLog:
2017-05-06  Sergio Durigan Junior  <sergiodj@redhat.com>

 	* configure.nat: Rearrange 'case' statements to match
	host before cpu.