proc/native: better error message when building on unsupported systems (#1883)

As we rearrange the code and the Go compiler changes the error message
returned by the compiler on unsupported architectures will change too,
making it un-googlable. Since the error message tends to be rather
obscure too this regularly confuses newbies.

This is an effort to make the error message for unsupported GOOS/GOARCH
combinations the same across all unsupported combinations and to make
it more user friendly.

Directories containing Go source code are supposed to contain a single
package. This property happens to be checked by cmd/go itself so it
will happen even before the syntax is fully checked and therefore has a
high probability of being the first (and only) error message being
print.

Here we take advantage of this by adding to the pkg/proc/native
directory a file with a bad package line that only gets compiled in on
unsupported GOOS/GOARCH combinations.

At present the error message for compiling Delve on unsupported systems
will be:

service/debugger/debugger.go:21:2: found packages native (proc.go) and your_operating_system_and_architecture_combination_is_not_supported_by_delve (support_sentinel.go) in $PATH_TO_DELVE/pkg/proc/native

pkg/proc/native/support_sentinel.go

+// This file is used to detect build on unsupported GOOS/GOARCH combinations.
+
+//+build !linux,!darwin,!windows,!freebsd linux,!amd64,!arm64 darwin,!amd64 windows,!amd64 freebsd,!amd64
+
+package your_operating_system_and_architecture_combination_is_not_supported_by_delve