If you have cgo and an assembly *.s file in the same package, the go command tries to
compile the assembly with `clang` instead of 6a.

See:

mac:~ bradfitz$ cd $GOPATH/src/foo

mac:foo bradfitz$ ls
bar.s foo.go

mac:foo bradfitz$ cat bar.s
TEXT    ·bar(SB),0,$0
        MOVQ $84, AX
        MOVQ AX, ret+0(FP)
        RET

mac:foo bradfitz$ cat foo.go
package main

/*

static int foo() {
return 42;
}

*/
import "C"

// In bar.s
func bar() int

func main() {
    println(C.foo())
    println(bar())
}
mac:foo bradfitz$ go list -json .
{
    "Dir": "/Users/bradfitz/src/foo",
    "ImportPath": "foo",
    "Name": "main",
    "Target": "/Users/bradfitz/bin/foo",
    "Stale": true,
    "Root": "/Users/bradfitz",
    "CgoFiles": [
        "foo.go"
    ],
    "SFiles": [
        "bar.s"
    ],
    "Imports": [
        "C"
    ],
    "Deps": [
        "errors",
        "runtime",
        "runtime/cgo",
        "sync",
        "sync/atomic",
        "syscall",
        "unsafe"
    ]
}
mac:foo bradfitz$ go install -x .
WORK=/var/folders/00/0g1n0000h01000cxqpysvccm001w6m/T/go-build192769161
mkdir -p $WORK/foo/_obj/
mkdir -p $WORK/foo/_obj/exe/
cd /Users/bradfitz/src/foo
/Users/bradfitz/go/pkg/tool/darwin_amd64/cgo -objdir $WORK/foo/_obj/ -- -I
$WORK/foo/_obj/ foo.go
/Users/bradfitz/go/pkg/tool/darwin_amd64/6c -F -V -w -I $WORK/foo/_obj/ -I
/Users/bradfitz/go/pkg/darwin_amd64 -o $WORK/foo/_obj/_cgo_defun.6 -D GOOS_darwin -D
GOARCH_amd64 $WORK/foo/_obj/_cgo_defun.c
clang -I . -g -O2 -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments
-fno-common -print-libgcc-file-name
clang -I . -g -O2 -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments
-fno-common -I $WORK/foo/_obj/ -o $WORK/foo/_obj/_cgo_main.o -c
$WORK/foo/_obj/_cgo_main.c
clang -I . -g -O2 -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments
-fno-common -I $WORK/foo/_obj/ -o $WORK/foo/_obj/_cgo_export.o -c
$WORK/foo/_obj/_cgo_export.c
clang -I . -g -O2 -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments
-fno-common -I $WORK/foo/_obj/ -o $WORK/foo/_obj/foo.cgo2.o -c $WORK/foo/_obj/foo.cgo2.c
clang -I . -g -O2 -fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments
-fno-common -I $WORK/foo/_obj/ -o $WORK/foo/_obj/bar.o -c ./bar.s
# foo
./bar.s:1:6: error: invalid character in input
TEXT ·bar(SB),0,$0
     ^
./bar.s:1:7: error: invalid character in input
TEXT ·bar(SB),0,$0
      ^
./bar.s:3:24: error: unexpected token in memory operand
        MOVQ AX, ret+0(FP)
                       ^