compiler.h: add __weak macro

Instead of writing out '__attribute__((weak))' use a shorter form.

Change-Id: If418a1d55052780077febd2d8f2089021f414b91
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25767
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

src/drivers/spi/spi-generic.c

@@ -15,6 +15,7 @@
  */
 
 #include <assert.h>
+#include <compiler.h>
 #include <spi-generic.h>
 #include <string.h>
 
@@ -116,7 +117,7 @@ unsigned int spi_crop_chunk(const struct spi_slave *slave, unsigned int cmd_len,
 	return min(ctrlr_max, buf_len);
 }
 
-void __attribute__((weak)) spi_init(void)
+void __weak spi_init(void)
 {
 	/* Default weak implementation - do nothing. */
 }

src/drivers/spi/tpm/tpm.c

@@ -17,6 +17,7 @@
 
 #include <arch/early_variables.h>
 #include <assert.h>
+#include <compiler.h>
 #include <commonlib/endian.h>
 #include <console/console.h>
 #include <delay.h>
@@ -64,7 +65,7 @@ void tpm2_get_info(struct tpm2_info *info)
 	*info = car_get_var(g_tpm_info);
 }
 
-__attribute__((weak)) int tis_plat_irq_status(void)
+__weak int tis_plat_irq_status(void)
 {
 	static int warning_displayed CAR_GLOBAL;