Try vm-actions instead of cross-platform-actions (#589)

This uses VirtualBox instead of xhyve.  It might be slower, but should
be more reliable.

Tests run on FreeBSD, NetBSD, and OpenBSD.  Only the FreeBSD ones are
passing at the moment, the others will keep running but aren't added as
dependencies for the action used to guard commits.

CMakeLists.txt

@@ -288,7 +288,7 @@ if(NOT SNMALLOC_HEADER_ONLY_LIBRARY)
 
   if(NOT (DEFINED SNMALLOC_LINKER_FLAVOUR) OR ("${SNMALLOC_LINKER_FLAVOUR}" MATCHES "^$"))
     # Linker not specified externally; probe to see if we can make lld work
-    set(CMAKE_REQUIRED_LINK_OPTIONS -fuse-ld=lld)
+    set(CMAKE_REQUIRED_LINK_OPTIONS -fuse-ld=lld -Wl,--icf=all)
     check_cxx_source_compiles("int main() { return 1; }" LLD_WORKS)
     if (LLD_WORKS)
       message(STATUS "Using LLD to link snmalloc shims")