ArmPkg/ArmSoftFloatLib: disable LTO build for GCC

ArmPkg/ArmSoftFloatLib: disable LTO build for GCC

Building ArmSoftFloatLib with LTO results in errors like

  .../bin/ld: softfloat.obj: plugin needed to handle lto object
  .../bin/ld: __aeabi_dcmpge.obj: plugin needed to handle lto object
  .../bin/ld: __aeabi_dcmplt.obj: plugin needed to handle lto object
  .../bin/ld: internal error ../../ld/ldlang.c 6299

This library is only linked by OpensslLib at the moment, and only
marginally used at runtime, so just disable LTO for it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>