The stack overflow checker in the linker uses the spadj field
to determine whether stack space will be large enough or not.
When spadj=0, the checker treats the function as a nosplit
and emits an error although the program is correct.

Also enable the stack checker in 8l.

Fixes #4316.

R=rsc, golang-dev
CC=golang-dev
https://golang.org/cl/6855088