cmd/ld: move symtab, ELF generation to portable code

More cleanup in preparation for fixing issue 4069.

This CL replaces the three nearly identical copies of the
asmb ELF code with a single asmbelf function in elf.c.

In addition to the ELF code movement, remove the elfstr
array in favor of a simpler lookup, and identify sections by
name throughout instead of computing fragile indices.

The CL also replaces the three nearly identical copies of the
genasmsym code with a single genasmsym function in lib.c.

The ARM linker still compiles and generates binaries,
but I haven't tested the binaries. They may not work.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7062047