sim: testsuite: initial support for OS-specific tests

sim: testsuite: initial support for OS-specific tests

We usually test against the newlib/libgloss environment, but for a
few ports that also support Linux apps, we want to test that logic
too.  A lot of the C code is written such that it works with either
newlib/libgloss or glibc/linux toolchains, but we have some tests
that end up being Linux-specific.  Cris has been using the target
tuple as a rough proxy for this (where cris*-*-elf is assumed to be
newlib/libgloss, and everything else is glibc/linux), but that is a
bit too rough, and it doesn't work in a multitarget build.

So lets create a few stub files that we can do compile tests with
to detect the different setups, and then let tests declare which
one they require (if they require any at all).