fix: compatible with lower clang version (#2252)

Summary:
* At least the Clang 11.0.0 can support "asm volatile inline", the lower version (e.g. 9.0.1) can only support "asm volatile".

Pull Request resolved: #2252

Reviewed By: yfeldblum

Differential Revision: D59772068

Pulled By: Orvid

fbshipit-source-id: cfba03e79aa3dc3437564772608971bceb3b8d83

folly/chrono/Hardware.h

@@ -91,7 +91,11 @@ inline std::uint64_t hardware_timestamp_measurement_start() noexcept {
   return ret;
 #elif defined(__GNUC__) && FOLLY_X64
   uint64_t ret = 0;
+#if !defined(__clang_major__) || __clang_major__ >= 11
   asm volatile inline(
+#else
+  asm volatile(
+#endif
       "lfence\n"
       "rdtsc\n" // loads 64-bit tsc into edx:eax
       "shl $32, %%rdx\n" // prep rdx for combine into rax
@@ -124,7 +128,11 @@ inline std::uint64_t hardware_timestamp_measurement_stop() noexcept {
   return ret;
 #elif defined(__GNUC__) && FOLLY_X64
   uint64_t ret = 0;
+#if !defined(__clang_major__) || __clang_major__ >= 11
   asm volatile inline(
+#else
+  asm volatile(
+#endif
       "rdtscp\n" // loads 64-bit tsc into edx:eax, clobbers ecx
       "shl $32, %%rdx\n" // prep rdx for combine into rax
       "or %%rdx, %[ret]\n" // combine rdx into rax