lib: Make log2() available in romstage on ARM, not just x86

On x86, log2() is defined as an inline function in arch/io.h. This is
a remnant of ROMCC, and forced us to not include clog2.c in romstage.
As a result, romstage on ARM has no log2().
Use the inline log2 only with ROMCC, but otherwise, use the one in
clog2.c.

Change-Id: Ifef2aa0a7b5a1db071a66f2eec0be421b8b2a56d
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/4681
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/lib.h

@@ -23,7 +23,7 @@
 #define __LIB_H__
 #include <stdint.h>
 
-#ifndef __PRE_RAM__ /* Conflicts with inline function in arch/io.h */
+#if !defined(__ROMCC__) /* Conflicts with inline function in arch/io.h */
 /* Defined in src/lib/clog2.c */
 unsigned long log2(unsigned long x);
 #endif