Prefer FOLLY_ATTR_WEAK over __attribute__((__weak__))

Summary: As per: https://github.com/facebook/folly/commit/ffbc96640b6db68a8e1b020b1048ab45b0699b23#commitcomment-30059248

Reviewed By: yfeldblum

Differential Revision: D9306902

fbshipit-source-id: 79eca471bf911c6791425432f537c6f2e7f99f49

folly/portability/Time.cpp

@@ -15,6 +15,8 @@
  */
 
 #include <folly/portability/Time.h>
+
+#include <folly/CPortability.h>
 #include <folly/Likely.h>
 
 #include <assert.h>
@@ -97,7 +99,7 @@ static int clock_thread_cputime(struct timespec* ts) {
   return 0;
 }
 
-__attribute__((weak)) int clock_gettime(clockid_t clk_id, struct timespec* ts) {
+FOLLY_ATTR_WEAK int clock_gettime(clockid_t clk_id, struct timespec* ts) {
   switch (clk_id) {
     case CLOCK_REALTIME: {
       auto now = std::chrono::system_clock::now().time_since_epoch();