New linux64 and nthread_linux64 Settings for SYBPLATFORM

linux64 and nthread_linux64 (for threaded applications) are now valid settings for the SYBPLATFORM environment variable that can be used for compiling Open Client and Open Server sample applications on Linux x86-64 64-bit. The existing linuxamd64 and nthread_linuxamd64 settings remain valid for the same use.