configure: Default to 'cc', not 'gcc'

configure: Default to 'cc', not 'gcc'

Default to 'cc' as our compiler, rather than 'gcc'. We used to have
to insist on gcc when we still kept the CPU env in a fixed global
register, but this is no longer necessary and we will now compile OK
on clang as well as gcc.  Using 'cc' should generally result in us
using the most standard and maintained system compiler for the
platform.  (For instance on newer MacOS X 'gcc' exists but is an
elderly compiler provided mostly for legacy reasons, and 'cc'
(which is clang) is definitely the better choice.) On Linux there
will generally be no user-visible change since cc will be gcc.

This changeover necessitates a slight reworking of how we set the
'cc' variable, because GNU cross toolchains generally provide a
'${cross_prefix}gcc' but not a '${cross_prefix}cc'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>