x/mobile: JNI library can't find __isoc99_sscanf on Android x86-64

[bemasc]

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

$ go version
go version go1.13.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/usr/local/google/home/bemasc/.cache/go-build"
GOENV="/usr/local/google/home/bemasc/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/usr/local/google/home/bemasc/work"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/google-golang"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/google-golang/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build596314756=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I built an Android app using a Go library and Gomobile. The Go library imports "net/http". When I run the app, it works fine on armv7, arm64, and x86, but it crashes as soon as the JNI module is imported on x86-64. This reproduces both in an x86-64 emulator and on x86-64 Chromebooks (e.g. Pixelbook, Pixel Slate) in the ARC++ environment.

java.lang.UnsatisfiedLinkError: dlopen failed: cannot locate symbol "__isoc99_sscanf" referenced by "/data/app/app.intra-1/lib/x86_64/libgojni.so"...
        at java.lang.Runtime.loadLibrary(Runtime.java:372)
        at java.lang.System.loadLibrary(System.java:1076)
        at go.LoadJNI.<clinit>(LoadJNI.java:23)
        at java.lang.Class.classForName(Native Method)
        at java.lang.Class.forName(Class.java:324)
        at java.lang.Class.forName(Class.java:285)
        at go.Seq.<clinit>(Seq.java:38)
        at go.Seq.touch(Seq.java:55)
        at tun2socks.Tun2socks.<clinit>(Tun2socks.java:11)

__isoc99_sscanf appears to be used by OPENSSL_cpuid_setup, a2i_ipadd, and ipv6_cb. It only appears in the x86-64 library, not any of the other architectures.

[bcmills]

CC @hyangah @steeve

[bemasc]

This was happening because my workstation's copy of go was built from the boringcrypto branch (and apparently this only affected the x86-64 build target). It would be nice if gomobile could separate the target stdlib from the system stdlib, but I wouldn't call that a bug.