cmd/go: installed c-shared libraries receiving ".a" suffix on Linux

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

go version go1.9.4 linux/amd64

Does this issue reproduce with the latest release?

Yes. Reproduced with 1.10

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

GOARCH="amd64"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build706877829=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

go install -buildmode=c-shared -x -a ./libpa

libpa contains a a C library declared in a main package.

What did you expect to see?

I expected the output file to be labeled libpa.so indicating it was a shared object

What did you see instead?

The output was labeled libpa.a indicating an archive file, but in actuality it was a shared object file.

file $GOPATH//pkg/linux_amd64_shared/<pkg dir>/libpa.a
libpa.a: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=31934079c8d80546780a304c28421f0d8228c9a4, not stripped

Additional Information

• go build -buildmode=c-shared -o libpa.so -x -a ./libpa produces the expected libpa.so output. This is how .so files are generated in cshared_test.go.
• go install -buildmode=c-archive -x -a ./libpa produces an archive file correctly named libpa.a
• init.go leaves the cfg.ExeSuffix empty for Linux machines when the c-shared option is selected (sets it to ".a" for c-archive).
• Passing GOEXE=".so" does not appear to overwrite the cfg.ExeSuffix.