lang/gforth: unbreak the build in the -Werror=int-conversion mode

Missing ecvt() function prototype was causing couple of warnings
which had become hard errors with recent versions of GCC (14+).
Getting rid of USE_GCC, while desirable, is not straightforward
and requires more work. While here, set the TEST_TARGET which is
part of the upstream's default ALL_TARGET, but we override that.

PR: 281238