I'm looking into this with the developers and have asked them to respond.
If it is a regular ARM FW and compiler environment, it should support long long (as 64bit on a 32 bit architecture, a double word, ARM has instructions for it to handle).
I would assume, yes, 64bit will work.
Just a questions of Soft- or HW floating point is used (an M3 inside might not support HW floating point engine, depends if RTL designers have enabled or not). A Soft-Floating-Point support via ARM runtime library should be possible instead. It needs just specific compiler command line option and to clarify if those can be specified on SDK/IDE.
ARM Cortex-M3 should be able to handle 64bit values. Try types as:
long long my64bitVar;
The only limitation: if you want to use printf: to print such value you would need actually:
printf("MyVar: %lld\n", my64bitVar).
It can happen that a stripped down runtime LIB is used which does not support such one on printf (or not any floating point).