cmd/ld: steps toward 386 host linking

cmd/ld: steps toward 386 host linking

- Introduce MaxAlign constant and use in data layout
and ELF section header.

- Allow up to 16-byte alignment for large objects
(will help Keith's hash changes).

- Emit ELF symbol for .rathole (global /dev/null used by 8c).

- Invoke gcc with -m32/-m64 as appropriate.

- Don't invoke gcc if writing the .o file failed.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7563045