cmd/ld: make mach-o sections match internal sections

This brings Mach-O generation more in line with ELF generation.

Having separate sections for the symtab and pclntab mean that we
can find them that way, instead of using the deprecated debug segments.
(And the host linker will keep separate sections for us, but probably
not the debug segments.)

R=ken2
CC=golang-dev
https://codereview.appspot.com/7688043