Added FreeBSD build instructions

[vladtr]

Added FreeBSD build instruction into docs. Its officially not supported, however it builds/runs/passes tests perfectly and instructions can be a useful for BSD users.

[spoonincode]

LLVM should actually not be required on freebsd; it's only required on Linux. The code that uses LLVM in mandel isn't clutched in on anything but Linux

mandel/CMakeLists.txt

Lines 72 to 76 in b5132e5

	if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64")
	list(APPEND EOSIO_WASM_RUNTIMES eos-vm-oc)
	# EOS VM OC requires LLVM, but move the check up here to a central location so that the EosioTester.cmakes
	# can be created with the exact version found
	find_package(LLVM REQUIRED CONFIG)

Did you try just using the latest default clang that comes with freebsd? Historically clang12+ w/ GNU's libstdc++ (how typically configured on Linux) breaks due to still unfixed defect in EOS VM:

• replace is_callable_v workaround with is_invocable_v; fixes build on clang12 EOSIO/eos-vm#210
  But that's just coincidentally broken at clang12 -- it doesn't have anything to do with llvm12+ being unsupported on Linux.

macOS seems to build fine with the latest Xcode which makes me wonder if latest clang may work with LLVM's libc++ on freebsd?

[vladtr]

By default vanilla installation of FreeBSD 13.1 comes with clang 13 and compilation fails with exact issue you pointed out :) So unfortunately clang needs to be rolled back there. FreeBSD port "llvm11" is just the easiest and most trouble-free way to install working clang11...