boost futex diff

This is based on a diff from Brad who noticed that the diff disabling
syscall(2) accidentally disabled futexes altogether. This diff reverts
to the state prior to that commit by taking the intended approach:
replace syscall(SYS_futex, ...) with a call to futex(2) itself.

The second part of the diff enables futexes for fiber.

I think this is an improvement over the current state. It went through a
bulk without fallout. Unless I hear objections, I plan on committing
this in a few days.

Index: Makefile
===================================================================
RCS file: /cvs/ports/devel/boost/Makefile,v
diff -u -p -r1.139 Makefile
--- Makefile 19 Dec 2023 06:14:25 -0000 1.139
+++ Makefile 21 Dec 2023 06:43:23 -0000
@@ -11,7 +11,8 @@ COMMENT-md= machine-dependent libraries
VERSION= 1.84.0
DISTNAME= boost_${VERSION:S/./_/g}
PKGNAME= boost-${VERSION}
-EPOCH = 0
+REVISION= 0
+EPOCH= 0
CATEGORIES= devel
SITES= https://boostorg.jfrog.io/artifactory/main/release/${VERSION}/source/
EXTRACT_SUFX= .tar.bz2
Index: patches/patch-boost_atomic_detail_futex_hpp
===================================================================
RCS file: /cvs/ports/devel/boost/patches/patch-boost_atomic_detail_futex_hpp,v
diff -u -p -r1.1 patch-boost_atomic_detail_futex_hpp
--- patches/patch-boost_atomic_detail_futex_hpp 28 Oct 2023 14:03:31 -0000 1.1
+++ patches/patch-boost_atomic_detail_futex_hpp 21 Dec 2023 07:22:52 -0000
@@ -1,44 +1,44 @@
Index: boost/atomic/detail/futex.hpp
--- boost/atomic/detail/futex.hpp.orig
+++ boost/atomic/detail/futex.hpp
-@@ -23,7 +23,7 @@
- #pragma once
-