Pass -latomic to linker when using clang

Summary:
clang compilation is failing due to a4fb1f8. In that commit I added a call to `std::atomic::is_lock_free` which was evidently relying on a compiler builtin only present in gcc.

Drawbacks to this fix are:

- users may need to install libatomic
- there might be cases where clang is used even though USE_CLANG is unset (e.g., when clang is the only available compiler). I didn't figure out how to add -latomic in those cases...

An alternative fix mentioned in http://lists.llvm.org/pipermail/llvm-bugs/2017-August/057263.html is using -stdlib=libc++ with clang.
Closes facebook#3769

Differential Revision: D7756261

Pulled By: ajkr

fbshipit-source-id: 26888300683fa9970ab5950239d1aa217e8efd49

build_tools/build_detect_platform

@@ -144,6 +144,8 @@ case "$TARGET_OS" in
         COMMON_FLAGS="$COMMON_FLAGS -DOS_LINUX"
         if [ -z "$USE_CLANG" ]; then
             COMMON_FLAGS="$COMMON_FLAGS -fno-builtin-memcmp"
+        else
+            PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -latomic"
         fi
         PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lpthread -lrt"
         # PORT_FILES=port/linux/linux_specific.cc
@@ -196,6 +198,8 @@ case "$TARGET_OS" in
         COMMON_FLAGS="$COMMON_FLAGS -DCYGWIN"
         if [ -z "$USE_CLANG" ]; then
             COMMON_FLAGS="$COMMON_FLAGS -fno-builtin-memcmp"
+        else
+            PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -latomic"
         fi
         PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lpthread -lrt"
         # PORT_FILES=port/linux/linux_specific.cc