js.core: simplifying cross-compilation with the v23 tool
js.core: simplifying cross-compilation with the v23 tool

This CL removes support for the "v23 xgo <label>" command and instead
provides a more idiomatic mechanism for cross-compiling Vanadium Go
binaries. Similar to Go cross-compilation, to cross-compile Vanadium
Go binaries that do not require custom toolchain, one simply runs:

GOARCH=<arch> GOOS=<os> v23 go ...

In addition, this CL introduces the V23_PROFILE environment variable
that can specify a development profile. If this variable is set, the
"v23 go ..."  command uses an environment variable setting expected by
the development profile identified by the value of the
variable. Currently, the following development profiles are supported:
"android", "arm", "java", and "nacl".

For instance, to cross-compile Vanadium Go binaries for arm (on
linux), one simply runs:

V23_PROFILE=arm v23 go ...

MultiPart: 2/2
Change-Id: I4a9ccb1215f2a6978910329561048dc8acb32a71

vanadium/js,vanadium/js,vanadium/js,vanadium/js