x86: add thread support

Thread support is added for the x86 architecture. Both
the local apic and the tsc udelay() functions have a
call to thread_yield_microseconds() so as to provide an
opportunity to run pending threads.

Change-Id: Ie39b9eb565eb189676c06645bdf2a8720fe0636a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3207
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/cpu/x86/tsc/delay_tsc.c

@@ -4,6 +4,7 @@
 #include <cpu/x86/tsc.h>
 #include <smp/spinlock.h>
 #include <delay.h>
+#include <thread.h>
 
 #if !defined(__PRE_RAM__)
 
@@ -176,6 +177,9 @@ void udelay(unsigned us)
 	unsigned long long current;
 	unsigned long long clocks;
 
+	if (!thread_yield_microseconds(us))
+		return;
+
 	start = rdtscll();
 	clocks = us;
 	clocks *= get_clocks_per_usec();