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>