5l, 6l, 8l: introduce sub-symbols

5l, 6l, 8l: introduce sub-symbols

Sub-symbols are laid out inside a larger symbol
but can be addressed directly.

Use to make Mach-O pointer array not a special case.

Will use later to describe ELF sections.

Glimpses of the beginning of ELF loading.

R=ken2
CC=golang-dev
https://golang.org/cl/2623043