[YottaDB/DB/YDB#850] Fix UTF-8 compilation on RHEL7 by using en_US.UTF-8 as a fallback locale if C.UTF-8 does not exist

A previous commit (6fc64b46) modified the Makefile to use the C.UTF-8 locale to compile the YDBEncrypt plugin's M code. This caused builds to fail on our in-house RHEL7 machine. After investigation, we discovered that the C.UTF-8 locale does not exist on RHEL7. It was added by Red Hat in version 8. Since RHEL7 is a supported operating system, the YDBEncrypt install needs to work there. The fix for this is to use en_US.UTF-8 as a fallback if the C.UTF-8 locale is not found.