Fixing bsr64 for MathLib (#492)

# Preface

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docs](https://github.com/microsoft/mu/blob/master/CONTRIBUTING.md) prior
to submitting the pull request. In particular,
[pull request
guidelines](https://github.com/microsoft/mu/blob/master/CONTRIBUTING.md#pull-request-best-practices).

## Description

When using GCC(-like) compilers to build MathLib, the `sqrt_d` function
will use built-in functions for bit scan reverse. However, the
calculation for 64 bit BSR is incorrectly using the built-in function
specifically for unsigned long, which is 32-bit for CLANGPDB and cause
the unit test to fail.

This change fixed the built-in function usage.

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## How This Was Tested

Unit test passed on QEMU SBSA platform built with CLANGPDB.

## Integration Instructions

N/A

MsCorePkg/Library/MathLib/MathLib.c

@@ -149,7 +149,7 @@ bsr64 (
   )
 {
  #if __GNUC__ > 3 // if we are using GCC take advantage of their builtins
-  return 64 - __builtin_clzl (value);
+  return 64 - __builtin_clzll (value);
 
  #elif _MSC_VER > 1500 // if we are using MS VS compiler 15 or greater
   unsigned long  result;

MsCorePkg/MsCorePkg.ci.yaml

@@ -109,7 +109,8 @@
             "SQRTUNSIGNED",
             "VARPOL",
             "SNP's",
-            "UEFI's"
+            "UEFI's",
+            "clzll"
         ],
         "AdditionalIncludePaths": [] # Additional paths to spell check relative to package root (wildcards supported)
     },