Build -md on riscv64, now with the required bits present in libcontext

Build-tested with icinga/core2 and net/powerdns_recursor, runtime tested
with a few test programs.  ok Brad (maintainer)

devel/boost/Makefile

@@ -1,6 +1,6 @@
-# $OpenBSD: Makefile,v 1.122 2021/11/07 10:30:35 jca Exp $
+# $OpenBSD: Makefile,v 1.123 2021/11/08 17:57:09 jca Exp $
 
-NOT_FOR_ARCHS-md = alpha hppa riscv64 sparc64
+NOT_FOR_ARCHS-md = alpha hppa sparc64
 
 COMMENT-main=	free peer-reviewed portable C++ source libraries
 COMMENT-md=	machine-dependent libraries for boost
@@ -11,7 +11,7 @@ DISTNAME=	boost_${VERSION:S/./_/g}
 PKGNAME-main=	boost-${VERSION}
 PKGNAME-md=	boost-md-${VERSION}
 REVISION-main=	2
-REVISION-md=	0
+REVISION-md=	1
 CATEGORIES=	devel
 MASTER_SITES=	https://boostorg.jfrog.io/artifactory/main/release/${VERSION}/source/
 EXTRACT_SUFX=	.tar.bz2

devel/boost/patches/patch-libs_context_build_Jamfile_v2

@@ -0,0 +1,25 @@
+$OpenBSD: patch-libs_context_build_Jamfile_v2,v 1.5 2021/11/08 17:57:10 jca Exp $
+
+Add support for clang on riscv64.
+
+Index: libs/context/build/Jamfile.v2
+--- libs/context/build/Jamfile.v2.orig
++++ libs/context/build/Jamfile.v2
+@@ -432,6 +432,17 @@ alias asm_sources
+      <toolset>gcc
+    ;
+ 
++alias asm_sources
++   : asm/make_riscv64_sysv_elf_gas.S
++     asm/jump_riscv64_sysv_elf_gas.S
++     asm/ontop_riscv64_sysv_elf_gas.S
++   : <abi>sysv
++     <address-model>64
++     <architecture>riscv
++     <binary-format>elf
++     <toolset>clang
++   ;
++
+ # S390X
+ # S390X/SYSV/ELF
+ alias asm_sources