Improve performance for Math.Abs (dotnet/coreclr#15823)
Improve performance for Math.Abs (dotnet/coreclr#15823)

* Improve perf for Math.Abs

* Inline Math.Abs

Signed-off-by: dotnet-bot-corefx-mirror <03c4cb6107ae99956a7681f2c97ff7104809a2bd@microsoft.com>

wtgodbe/corefx,ptoonen/corefx,shimingsg/corefx,ViktorHofer/corefx,ptoonen/corefx,Ermiar/corefx,ptoonen/corefx,ViktorHofer/corefx,wtgodbe/corefx,mmitche/corefx,shimingsg/corefx,Jiayili1/corefx,mmitche/corefx,Ermiar/corefx,wtgodbe/corefx,zhenlan/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ericstj/corefx,ViktorHofer/corefx,shimingsg/corefx,mmitche/corefx,zhenlan/corefx,Ermiar/corefx,ravimeda/corefx,Jiayili1/corefx,Ermiar/corefx,wtgodbe/corefx,shimingsg/corefx,BrennanConroy/corefx,Jiayili1/corefx,ptoonen/corefx,ptoonen/corefx,ravimeda/corefx,ericstj/corefx,ravimeda/corefx,ericstj/corefx,ptoonen/corefx,zhenlan/corefx,Ermiar/corefx,shimingsg/corefx,ravimeda/corefx,mmitche/corefx,BrennanConroy/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,mmitche/corefx,zhenlan/corefx,zhenlan/corefx,ericstj/corefx,Jiayili1/corefx,Jiayili1/corefx,Ermiar/corefx,mmitche/corefx,wtgodbe/corefx,shimingsg/corefx,zhenlan/corefx,Jiayili1/corefx,zhenlan/corefx,ravimeda/corefx,ViktorHofer/corefx,ravimeda/corefx,ptoonen/corefx,ericstj/corefx,Jiayili1/corefx,BrennanConroy/corefx,Ermiar/corefx,mmitche/corefx,ravimeda/corefx,wtgodbe/corefx,wtgodbe/corefx,ericstj/corefx