gb should fail early with unknown/unsupported GOOS or GOARCH value

[davecheney]

lucky(~/src/github.com/constabulary/gb) % env GOOS=386 go build
cmd/go: unsupported GOOS/GOARCH pair 386/amd64
lucky(~/src/github.com/constabulary/gb) % env GOOS=386 gb build
compile: unknown TLS base register for 0
# errors
# runtime/internal/sys
intrinsics_stubs.go:9:6: missing function body for "Ctz64"
intrinsics_stubs.go:10:6: missing function body for "Ctz32"
intrinsics_stubs.go:11:6: missing function body for "Bswap64"
intrinsics_stubs.go:12:6: missing function body for "Bswap32"
compile: unknown TLS base register for 0
# sync/atomic
# syscall
syscall.go:83:11: undefined: Timespec
compile: unknown TLS base register for 0
# math
j0.go:106:11: undefined: Sincos
j0.go:195:11: undefined: Sincos
j1.go:103:11: undefined: Sincos
j1.go:180:11: undefined: Sincos
# unicode/utf8
compile: unknown TLS base register for 0
# math/bits
compile: unknown TLS base register for 0
# container/list
compile: unknown TLS base register for 0
# crypto/subtle
compile: unknown TLS base register for 0
# crypto/internal/cipherhw
compile: unknown TLS base register for 0
compile: unknown TLS base register for 0
# encoding
# vendor/golang_org/x/crypto/curve25519
internal/race
github.com/constabulary/gb/internal/version
internal/nettrace
runtime/internal/atomic
compile: unknown TLS base register for 0
# unicode/utf16
compile: unknown TLS base register for 0
# unicode
FATAL: command "build" failed: exit status 1