Unbreak on riscv64

ok ajacoutot@ (maintainer) sthen@

www/webkitgtk4/patches/patch-Source_WTF_wtf_PlatformHave_h

@@ -0,0 +1,18 @@
+Undo parts of upstream commit 6c2615dd66a12e9a10d7867241cc3bedfb2b2473 [0]
+Machine context definitions were only added for Linux.
+[0] https://github.com/WebKit/WebKit/commit/6c2615dd66a12e9a10d7867241cc3bedfb2b2473
+
+Index: Source/WTF/wtf/PlatformHave.h
+--- Source/WTF/wtf/PlatformHave.h.orig
++++ Source/WTF/wtf/PlatformHave.h
+@@ -233,6 +233,10 @@
+ #define HAVE_MACHINE_CONTEXT 1
+ #endif
+ 
++#if OS(OPENBSD) && CPU(RISCV64)
++#undef HAVE_MACHINE_CONTEXT
++#endif
++
+ #if OS(DARWIN) || (OS(LINUX) && defined(__GLIBC__) && !defined(__UCLIBC__) && !CPU(MIPS))
+ #define HAVE_BACKTRACE 1
+ #endif