cheri: malloc: Set fixed mmap_threshold for capability narrowing

cheri: malloc: Set fixed mmap_threshold for capability narrowing

Capability narrowing requires extra alignment for large allocations,
this can cause significant heap fragmentation so ensure large
allocations use a single mmap and unmaped on free. The fragmentation
is bug 14581.

This patch disables dynamic mmap threshold when cap_narrowing is
enabled and uses a threshold of 256 kbytes on morello (which means
at most 64 byte alignment requirement on the heap). The dynamic
threshold could quickly grow very big and stay there making the
fragmentation issue common.