use jl_isspace instead of isspace

due to Windows XP undefined behavior with unicode signed char values > 127

Fixes #7303, allowing Julia to run again on Windows XP (won't build on XP, but I tested a build from Win7 and it can be installed and run now), at the cost of duplicating some rather basic functionality. `isspace` may also be locale-dependent, which this should prevent from being a problem.

@vtjnash found the problem, hope I'm not stepping on his toes by proposing a fix.