Updates for go1.18beta1 (#2831)

* go.mod: update golang.org/x/tools to v0.1.8

Fixes TestGeneratedDoc on go1.18

* TeamCity: bump test matrix

Add 1.18 to test matrix. Remove 1.15 from test matrix and from support range.

* proc,tests: update for regabi on arm64 and 386

Make sure that stacktrace registers always contain the PC register of
the current frame, even though the debug_frame rules might not specify
it on architectures that use a link register.
The PC register is needed to look up loclist entries for variable
evaluation.

* goversion: bump maximum supported Go version to 1.18

* proc: disable asyncpreempt on linux/arm64

Asyncpreempt on linux/arm64 can sometimes restart a sequence of
instructions which will make breakpoint appear to be hit twice in some
cases.

_scripts/test_linux.sh

@@ -21,10 +21,6 @@ function getgo {
 }
 
 if [ "$version" = "gotip" ]; then
-	# TODO: remove this
-	if [ "$arch" != "amd64" ]; then
-		exit 0
-	fi
 	echo Building Go from tip
 	getgo $(curl https://go.dev/VERSION?m=text)
 	export GOROOT_BOOTSTRAP=$GOROOT