UefiCpuPkg/BaseUefiCpuLib: Use NASM read-only data section name

https://bugzilla.tianocore.org/show_bug.cgi?id=556

NASM requires read-only data sections to use the section
name .rodata.  This fix changes .rdata to .rodata.

The build failure from use of .rdata is seen when using
the XCODE5 tool chain.

Section "7.8.1 macho extensions to the SECTION Directive"
of the NASM documentation at http://www.nasm.us/doc/
describes the section name requirements.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Andrew Fish <afish@apple.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>

UefiCpuPkg/Library/BaseUefiCpuLib/Ia32/InitializeFpu.nasm

@@ -1,6 +1,6 @@
 ;------------------------------------------------------------------------------
 ;*
-;*   Copyright (c) 2016, Intel Corporation. All rights reserved.<BR>
+;*   Copyright (c) 2016 - 2017, Intel Corporation. All rights reserved.<BR>
 ;*   This program and the accompanying materials
 ;*   are licensed and made available under the terms and conditions of the BSD License
 ;*   which accompanies this distribution.  The full text of the license may be found at
@@ -12,7 +12,7 @@
 ;*
 ;------------------------------------------------------------------------------
 
-    SECTION .rdata
+    SECTION .rodata
 
 ;
 ; Float control word initial value: