Compiler suggested cleanup
Fix to avoid excessive compiler optimization of MicroVAX II ROM access spinwait.  Found by Cory Smelosky and Jordi Guillaumes Pons.

Used same solution as the MicroVAX 3900 ROM access logic suggested by Sergy Oboguev in April 2012.
MAKEFILE: Added support to attempt to use the OS provided cc if gcc isn't available or otherwise specified by GCC= on the make command line.