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/lib/timestamp.c

@@ -361,7 +361,7 @@ ROMSTAGE_CBMEM_INIT_HOOK(timestamp_sync_cache_to_cbmem)
 RAMSTAGE_CBMEM_INIT_HOOK(timestamp_sync_cache_to_cbmem)
 
 /* Provide default timestamp implementation using monotonic timer. */
-uint64_t  __attribute__((weak)) timestamp_get(void)
+uint64_t  __weak timestamp_get(void)
 {
 	struct mono_time t1, t2;
 
@@ -375,7 +375,7 @@ uint64_t  __attribute__((weak)) timestamp_get(void)
 }
 
 /* Like timestamp_get() above this matches up with microsecond granularity. */
-int __attribute__((weak)) timestamp_tick_freq_mhz(void)
+int __weak timestamp_tick_freq_mhz(void)
 {
 	return 1;
 }