refactor com.sun.org.apache.xalan.internal.xsltc.runtime.BasicLibrary.substringF() variants

These two methods could be refactored to avoid unnecessary calls to Math.round(double). In workloads which make frequent calls to these two methods could experience high sys CPU time if Math.round(double) if what Math.round(double) calls invokes native methods.

com.sun.org.apache.xalan.internal.xsltc.runtime.BasicLibrary.substringF(String,double,double)
com.sun.org.apache.xalan.internal.xsltc.runtime.BasicLibrary.substringF(String,double)

It is also possible a modern JVM's JIT compiler could better optimize a refactored version.