libroot: Force __cxa_pure_virtual to be linked in.

In GCC 8 builds it is by default, in GCC 11 builds it is weakly
defined, which runtime_loader does not yet support properly,
so we force it to be included.

src/system/libroot/Jamfile

@@ -77,6 +77,12 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 		SetVersionScript $(libroot) : libroot_versions ;
 		SetVersionScript $(librootDebug) : libroot_versions ;
 
+		if $(architecture) != x86_gcc2 {
+			# force __cxa_pure_virtual to be linked instead of weak
+			# TODO: remove once we handle weak symbols correctly!
+			LINKFLAGS on $(libroot) += -Wl,-u,__cxa_pure_virtual ;
+		}
+
 		SharedLibrary $(libroot)
 			:
 			libroot_init.c