Create fimrware/asm directory for assembly optimized stuff.

This dir is suitable for stuff that doesn't fit the target tree, e.g. because
it also builds on hosted or otherwise. It also has a generic subfolder for
fallback C implementations so that not all archs need to provide asm files.

SOURCES should only contain "foo.c" where foo.c includes the specific
<arch>/foo.c files from the subdirs using the preprocessor. This way automatic
selection of asm versions or generic C verion is possible.

For the start, the thread support files are moved, since ASM threads can
be used on hosted platforms as well. Since core_sleep() remains platform
specific it's moved to the corresponding system.h headers.

Change-Id: Iebff272f3407a6eaafeb7656ceb0ae9eca3f7cb9

firmware/asm/thread.c

@@ -0,0 +1,21 @@
+#if defined(CPU_ARM)
+  #include "arm/thread.c"
+#elif defined(CPU_COLDFIRE)
+  #include "m68k/thread.c"
+#elif CONFIG_CPU == SH7034
+  #include "sh/thread.c"
+#elif defined(CPU_MIPS)
+  #include "mips/thread.c"
+#else
+
+/* generic thread.c */
+
+#if defined(HAVE_WIN32_FIBER_THREADS)
+  #include "thread-win32.c"
+#elif defined(HAVE_SIGALTSTACK_THREADS)
+  #include "thread-unix.c"
+#else
+  #error Missing thread impl
+#endif
+
+#endif