lib: Add asmlinkage attribute to bootblock_main_with_timestamp

Add asmlinkage to bootblock_main_with_timestamp so that it may be called
directly from the assembly code.

TEST=Build for Amenia and Galileo Gen2

Change-Id: Iefb8e5c1ddce2ec495b9272966b595d5adcebc1c
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/15125
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/bootblock_common.h

@@ -16,6 +16,7 @@
 #ifndef __BOOTBLOCK_COMMON_H
 #define __BOOTBLOCK_COMMON_H
 
+#include <arch/cpu.h>
 #include <main_decl.h>
 #include <stdint.h>
 
@@ -36,6 +37,6 @@ void bootblock_soc_init(void);
  * This function assumes that the timer has already been initialized, so it
  * does not call init_timer().
  */
-void bootblock_main_with_timestamp(uint64_t base_timestamp);
+void asmlinkage bootblock_main_with_timestamp(uint64_t base_timestamp);
 
 #endif	/* __BOOTBLOCK_COMMON_H */

src/lib/bootblock.c

@@ -29,7 +29,7 @@ __attribute__((weak)) void bootblock_soc_early_init(void) { /* do nothing */ }
 __attribute__((weak)) void bootblock_soc_init(void) { /* do nothing */ }
 __attribute__((weak)) void bootblock_mainboard_init(void) { /* do nothing */ }
 
-void bootblock_main_with_timestamp(uint64_t base_timestamp)
+void asmlinkage bootblock_main_with_timestamp(uint64_t base_timestamp)
 {
 	/* Initialize timestamps if we have TIMESTAMP region in memlayout.ld. */
 	if (IS_ENABLED(CONFIG_COLLECT_TIMESTAMPS) && _timestamp_size > 0)