liblink, cmd/ld: reenable nosplit checking and test
The new code is adapted from the Go 1.2 nosplit code,
but it does not have the bug reported in issue 7623:
g% go run nosplit.go
g% go1.2 run nosplit.go
BUG
rejected incorrectly:
main 0 call f; f 120
linker output:
# _/tmp/go-test-nosplit021064539
main.main: nosplit stack overflow
120 guaranteed after split check in main.main
112 on entry to main.f
-8 after main.f uses 120
g%
Fixes #6931.
Fixes #7623.
LGTM=iant
R=golang-codereviews, iant, ality
CC=golang-codereviews, r
https://golang.org/cl/
88190043