Fix bug in llvm::sys::argumentsFitWithinSystemLimits().

This patch fixes a subtle bug in the UNIX implementation of
llvm::sys::argumentsFitWithinSystemLimits() regarding the misuse of a static
variable. This bug causes our cached number that stores the system command line
maximum length to be halved after each call to the function. With a sufficient
number of calls to this function, it will eventually report any given command
line string to be over system limits.

Patch by Rafael Auler.