[SystemZ][z/OS] Fix warning caused by umask returning a signed integer type
On z/OS, umask() returns an int because mode_t is type int, however it is being compared to an unsigned int. This patch fixes the following warning we see when compiling Path.cpp.
comparison of integers of different signs: 'const int' and 'const unsigned int'
Reviewed By: muiez
Differential Revision: https://reviews.llvm.org/D102326