rts/Capability.c: fix crash in -threaded mode on UNREG build

UNREG mode has quite nasty invariant to maintain:
    capabilities[0] == &MainCapability

and it's a non-heap memory, while other
capabilities are dynamically allocated.

Issue #8748

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Austin Seipp <austin@well-typed.com>
(cherry picked from commit ebace696)