Gmp.mpz: Restored the 1GB memory cut off in pow().
Gmp does NOT handle out of memory gracefully (GNU MP 6.1.0 manual):
There's currently no defined way for the allocation functions to
recover from an error such as out of memory, they must terminate
program execution. A longjmp or throwing a C++ exception will have
undefined results. This may change in the future.
Fixes multiple testsuite failures.