libtool: Fix support for NIOS2 processor

libtool: Fix support for NIOS2 processor

When building for a nios2 system, the $host tuple starts with "nios2-"
which is caught by the some of the greedy checks for OS/2 in libtool.
In particular, the *os2* branches of switch statements that only want
to match the OS setting end up matching all nios2 targets, which
results in incorrect behavior.

Switch to use $host_os instead of $host and tweak the patterns to
match to avoid this problem for nios2.

* build-aux/ltmain.in: Switch some $host checks to $host_os.