cheri: malloc: Initial capability narrowing support

cheri: malloc: Initial capability narrowing support

Public interfaces return pointers with narrow bounds, this internally
requires bumping the size and alignment requirement of allocations so
the bounds are representible.

When pointers with narrow bounds need to be turned back to have wide
bounds (free, realloc), the pointer is rederived from DDC. (So this
patch relies on DDC to cover all heap memory with RW permission.)

Allocations above the mmap threshold waste memory for alignment and
realloc often falls back to the inefficient alloc, copy, free sequence
instead of mremap or other inplace solution.