os: TestStartProcess and TestHostname fail on android

[mikioh]

See http://build.golang.org/log/8f8c15bb002fa2ba25799e34e155095462700a47

go_android_exec: adb shell mkdir -p /data/local/tmp/os.test-7203
go_android_exec: adb push /tmp/go-build611040176/os/_test/os.test /data/local/tmp/os.test-7203/os.test-7203-tmp
199 KB/s (2565204 bytes in 12.544s)
go_android_exec: adb shell cp '/data/local/tmp/os.test-7203/os.test-7203-tmp' '/data/local/tmp/os.test-7203/os.test-7203'
go_android_exec: adb shell rm '/data/local/tmp/os.test-7203/os.test-7203-tmp'
go_android_exec: adb shell export TMPDIR="/data/local/tmp/os.test-7203"; export GOROOT="/data/local/tmp/goroot"; export GOPATH="/data/local/tmp/gopath"; cd "/data/local/tmp/goroot/src/os"; '/data/local/tmp/os.test-7203/os.test-7203' -test.short=true -test.timeout=4m0s; echo -n exitcode=$?
--- FAIL: TestStartProcess (0.00s)
    os_test.go:780: StartProcess: fork/exec /bin/pwd: no such file or directory
--- FAIL: TestHostname (0.01s)
    os_test.go:1210: fork/exec /bin/hostname: no such file or directory
FAIL
exitcode=1go_android_exec: adb shell rm -rf /data/local/tmp/os.test-7203
FAIL    os  13.867s

There needs to have some proper access methods to android-specific system properties.

[gopherbot]

CL https://golang.org/cl/11223 mentions this issue.