Force argv to be an array of long pointers on VMS

Reverts commit 087ca80a

Instead of battling the odd format of argv given to main() in default
P64 mode, tell the compiler to make it an array of 64-bit pointers
when compiling in P64 mode.

A note is added in NOTES.VMS regarding minimum DEC C version.

Reviewed-by: Andy Polyakov <appro@openssl.org>