[libc++] Improve Lit's buildhost=XXXX feature on a few platforms

[ldionne]

Use new buildhost feature in transitive includes test.

libcxx/utils/libcxx/test/features.py

DEFAULT_FEATURES += [
   Feature(name='netbsd', when=lambda cfg: '__NetBSD__' in compilerMacros(cfg)),
   Feature(name='freebsd', when=lambda cfg: '__FreeBSD__' in compilerMacros(cfg))
 ]
 
 # Add features representing the build host platform name.
 # The build host could differ from the target platform for cross-compilation.
 DEFAULT_FEATURES += [
   Feature(name='buildhost={}'.format(sys.platform.lower().strip())),
-  # sys.platform can be represented by "sub-system" on Windows host, such as 'win32', 'cygwin', 'mingw' & etc.
-  # Here is a consolidated feature for the build host plaform name on Windows.
-  Feature(name='buildhost=windows', when=lambda cfg: platform.system().lower().startswith('windows'))
+  # sys.platform can often be represented by a "sub-system", such as 'win32', 'cygwin', 'mingw', freebsd13 & etc.
+  # We define a consolidated feature on a few platforms.
+  Feature(name='buildhost=windows', when=lambda cfg: platform.system().lower().startswith('windows')),
+  Feature(name='buildhost=freebsd', when=lambda cfg: platform.system().lower().startswith('freebsd')),
+  Feature(name='buildhost=aix', when=lambda cfg: platform.system().lower().startswith('aix'))
 ]
 
 # Detect whether GDB is on the system, has Python scripting and supports
 # adding breakpoint commands. If so add a substitution to access it.
 def check_gdb(cfg):
   gdb_path = shutil.which('gdb')
   if gdb_path is None:
     return False