Fix some sign compare warnings.

This change changes kmp_bstate.old_tid to sign integer instead of unsigned integer.
It also defines two new macros KMP_NSEC_PER_SEC and KMP_USEC_PER_SEC which lets us take
control of the sign (we want them to be longs).  Also, in kmp_wait_release.h, the byteref()
function's return type is changed from char to unsigned char.

llvm-svn: 239057