tests/tcg/configure.sh: tweak quoting of target_compiler

tests/tcg/configure.sh: tweak quoting of target_compiler

If you configure the host compiler with a multi-command stanza like:

  --cc="ccache gcc"

then the configure.sh machinery falls over with confusion. Work around
this by ensuring we correctly quote so where we need a complete
evaluation we get it. Of course the has() check needs single variable
so we need to unquote that. This does mean it essentially checks that
just the ccache command exits but if we got past that step we still
check the compiler actually does something.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Message-Id: <20210527160319.19834-4-alex.bennee@linaro.org>