auto/cc: Use portable/POSIX 'command -v' instead of 'which'

auto/cc: Use portable/POSIX 'command -v' instead of 'which'

'which' is not a portable utility as it is not specified by POSIX. Since
auto/cc is already a shell script, use the more direct and portable
'command' builtin to detect $CC.

There are two bugs linked here. The first one is a downstream report of
this issue. The second one is more general information on why 'which'
usage is an issue and should be avoided.

Bug: https://bugs.gentoo.org/969288
Bug: https://bugs.gentoo.org/646588
Signed-off-by: Zurab Kvachadze <zurabid2016@gmail.com>