Adjust sandbox linker script to expect CRT parts for the CHERI
runtime to be named crtbeginC.o and crtendC.o; these will bracket
constructors [and destructors] found in the .ctors and .dtors ELF
sections.
Add crtbeginC.c and crtendC.c, which bracket constructors and destructors
inserted by the compiler in the .ctor and .dtor sections.  The symbols in
these files allow the runtime to [in principle] iterate over the function
pointers for required functions by placing '-1' pointers before and after
each array.