Cray: Enable Cray compiler wrapper detection on all platforms

Customer reported problems detecting the Cray compiler wrapper on their Apollo80 system. We were checking for the __CRAYXC and __CRAYXE predefined macros. These macros reflect the platform that the compiler wrapper is running on, i.e. Cray XC and Cray XE machines. Naturally, this didn't work on Apollo80.

This commit uses the __CRAYXT_COMPUTE_LINUX_TARGET macro. The Cray cc/CC/ftn wrappers always define this macro on the command line. This macro has been in use for many years, and is believed to be a reliable way to detect current and older Cray compiler wrappers.

Fixes: #21904 (closed)
Backport: release