cmd/go: provide better error message when cross-compiling toolchain is required

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

510fb6397dfc93067dc90d42c58dfc5f8b995285: tip between 1.7rc1 and 1.7rc2

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

GOARCH="amd64"
GOBIN="/Users/josh/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/josh"
GORACE=""
GOROOT="/Users/josh/go/tip"
GOTOOLDIR="/Users/josh/go/tip/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/pp/0010hzld5b12h_92j5th082r0000gn/T/go-build629103623=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

1. What did you do?

GOOS=android GOARCH=amd64 go install std cmd

1. What did you expect to see?

Success.

1. What did you see instead?

# cmd/addr2line
warning: unable to find runtime/cgo.a
/Users/josh/go/tip/pkg/tool/darwin_amd64/link: running clang failed: exit status 1
ld: unknown option: -z
clang: error: linker command failed with exit code 1 (use -v to see invocation)

# cmd/asm
warning: unable to find runtime/cgo.a
/Users/josh/go/tip/pkg/tool/darwin_amd64/link: running clang failed: exit status 1
ld: unknown option: -z
clang: error: linker command failed with exit code 1 (use -v to see invocation)

and so on

Running GOOS=android GOARCH=amd64 go install -ldflags=-v std cmd:

# cmd/cgo
HEADER = -H4 -T0x401000 -D0x0 -R0x1000
searching for runtime.a in $WORK/runtime.a
searching for runtime.a in /Users/josh/go/tip/pkg/android_amd64_shared/runtime.a
searching for runtime/cgo.a in $WORK/runtime/cgo.a
searching for runtime/cgo.a in /Users/josh/go/tip/pkg/android_amd64_shared/runtime/cgo.a
warning: unable to find runtime/cgo.a
 0.00 deadcode
 0.03 pclntab=599713 bytes, funcdata total 119620 bytes
 0.03 dodata
 0.04 dwarf
 0.07 symsize = 0
 0.16 reloc
 0.17 asmb
 0.17 codeblk
 0.17 datblk
 0.18 sym
 0.18 symsize = 122040
 0.19 symsize = 122088
 0.19 dwarf
 0.21 headr
host link: "clang" "-m64" "-gdwarf-2" "-Wl,-z,relro" "-pie" "-o" "/var/folders/pp/0010hzld5b12h_92j5th082r0000gn/T/go-build962217633/cmd/cgo/_obj/exe/a.out" "-rdynamic" "-Qunused-arguments" "/var/folders/pp/0010hzld5b12h_92j5th082r0000gn/T/go-link-534529285/go.o"
/Users/josh/go/tip/pkg/tool/darwin_amd64/link: running clang failed: exit status 1
ld: unknown option: -z
clang: error: linker command failed with exit code 1 (use -v to see invocation)

and so on

This also causes GOOS=android GOARCH=amd64 ./bootstrap.bash to fail.

Maybe this is expected (?), but it doesn't seem right to me.