Improve String.EndsWith for OrdinalIgnoreCase

[EgorBo]

Use Equals instead of Compare. The same method is used for String.StartsWith

static readonly string TestStr = typeof(object).Assembly.Location;

[Benchmark]
public bool EndsWith_small() => 
    TestStr.EndsWith(".DLL", StringComparison.OrdinalIgnoreCase);

[Benchmark]
public bool EndsWith_large() => 
    TestStr.EndsWith("system.private.corelib.dll", StringComparison.OrdinalIgnoreCase);

Method	Toolchain	Mean	Error	StdDev	Ratio
EndsWith_small	\Core_Root\corerun.exe	4.502 ns	0.0076 ns	0.0064 ns	0.54
EndsWith_small	\Core_Root_base\corerun.exe	8.368 ns	0.0113 ns	0.0106 ns	1.00
EndsWith_large	\Core_Root\corerun.exe	12.669 ns	0.0456 ns	0.0404 ns	0.60
EndsWith_large	\Core_Root_base\corerun.exe	21.006 ns	0.0429 ns	0.0358 ns	1.00

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.