cmd/link: TestRuntimeTypeAttr fails when gcc is unavailable

[iWdGo]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

Go 1.11beta2 as bootstrap and 
go version devel +e5b13401c6 Wed Jul 25 23:53:54 2018 +0000 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\()\AppData\Local\go-build
set GOEXE=.exe
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\()\Documents\Google\
set GOPROXY=
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\C()osta\AppData\Local\Temp\go-build629604138=/tmp/go-build -gno-record-gcc-switches

What did you do?

>all.bat

If possible, provide a recipe for reproducing the error.
A complete runable program is good.
A link on play.golang.org is best.

What did you expect to see?

ok cmd/link/internal/ld 114.969s

What did you see instead?

--- FAIL: TestRuntimeTypeAttr (23.85s)
    --- FAIL: TestRuntimeTypeAttr/flags=-ldflags=-linkmode=external (14.24s)
        dwarf_test.go:104: build: # command-line-arguments
            loadinternal: cannot find runtime/cgo
            C:\Users\Costa\Documents\Google\golang\go\pkg\tool\windows_amd64\link.exe: running gcc failed: exec: "gcc": executable file not found in %PATH%


        dwarf_test.go:105: build error: exit status 2
FAIL
FAIL    cmd/link/internal/ld    138.075s

I'll submit a small fix as the failing test is timing out on gcc instead of checking its availability as elsewhere using testenv.MustHaveCGO(t).

[gopherbot]

Change https://golang.org/cl/126075 mentions this issue: cmd/link: added test of availability of gcc