ArmPkg/ArmDisassemblerLib: replace AsciiStrCat() with AsciiStrCatS()

ArmPkg/ArmDisassemblerLib: replace AsciiStrCat() with AsciiStrCatS()

AsciiStrCat() is deprecated / disabled under the
DISABLE_NEW_DEPRECATED_INTERFACES feature test macro.

The "Str" variable serves no particular purpose in the MRegList() and
ThumbMRegList() functions; replace it with the pointed-to "mMregListStr" /
"mThumbMregListStr" global variable (as appropriate), so that the new
AsciiStrCatS() calls are as clear as possible.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Michael Zimmermann <sigmaepsilon92@gmail.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=164
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=165
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>