libfolly: Add package for folly library

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 <redacted>