aarch64: morello: add purecap start code

aarch64: morello: add purecap start code

Written in C so the self relocation code in crt1.o is easier to
maintain.

The purecap ELF entry is special: passes separate argc, argv, envp,
auxv in registers instead of on the stack.

For each module there will be separate RW and RX capabilities that
cover the writable and all load segments respectively. The relative
reloc processing code is prepared for such separate capabilities.

The static link detection (for self relocation) is not ideal, it
relies on relocations that don't work in PIC, so it is ifdefed out
for Scrt1.o. (Currently adrp of undefined weak symbol is not fixed
up by the linker to be 0 so we use movz to detect the presence of
__rela_dyn_start.)