The signal mask argument is passed as a struct with a pointer and size
in the linux syscall abi, but the types used in glibc were wrong for
CHERI due to an x32 specific hack.

This reverts commit 3a69d7f241ab9f738479e039bd6eecf433868375.

This reverts commit dc8f9560104f0803e53fa23bfd011aa9878d5a97.

malloc/tst-malloc-backtrace tests heap corruption.
malloc/tst-dynarray uses malloc_debug wrappers that access internals.

Add more cap_ hooks to implement narrowing without depending on a
global capability covering the heap.  Either recording every
narrowed capability in a lookup table or recording every mapping
used for the heap are supported.  The morello implmentation uses
a lookup table for now.

The lookup table adds memory overhead, failure paths and locks.
Recording and removing entries from the lookup table must be done
carefully in realloc so on failure the old pointer is usable and
on success the old pointer is immediately reusable concurrently.
The locks require fork hooks so malloc works in multi-threaded
fork child.