libfolly: Add package for folly library

folly is an open-source C++ library developed and used at Facebook.
Adding this package will enable adding further packages that depend on
this.

Depends on PR #7098, PR #7101, PR #7126, PR #7877

Maintainer: me
Compile tested: openwrt-18.06 for ipq40xx, ipq806x, x86 and ar71xx
Run tested: Tested on devices running on the above architectures.
Verified that dependent packages can successfully build and run using
the libfolly.so shared library.

Signed-off-by: Amol Bhave <ambhave@fb.com>

libs/libfolly/patches/101-remove-undefined-dcheck.patch

@@ -0,0 +1,12 @@
+Index: folly-2019.05.06.00/folly/stats/detail/BufferedStat-defs.h
+===================================================================
+--- folly-2019.05.06.00.orig/folly/stats/detail/BufferedStat-defs.h
++++ folly-2019.05.06.00/folly/stats/detail/BufferedStat-defs.h
+@@ -74,7 +74,6 @@ void BufferedStat<DigestT, ClockT>::doUp
+     TimePoint now,
+     const std::unique_lock<SharedMutex>& g,
+     UpdateMode updateMode) {
+-  DCHECK(g.owns_lock());
+   // Check that no other thread has performed the slide after the check
+   auto oldExpiry = expiry_.load(std::memory_order_relaxed).tp;
+   if (now > oldExpiry || updateMode == UpdateMode::Now) {