configure: Diagnose broken linkers directly

Currently if the user's compiler works for creating .o files but
their linker is broken such that compiling an executable from a
C file does not work, we will report a misleading error message
about the compiler not supporting __thread (since that happens
to be the first test we run which requires a working linker).
Explicitly check that compile_prog works as well as compile_object,
so that people whose toolchain setup is broken get a more helpful
error message.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
