MdeModulePkg/DxeCore: base code protection on permission attributes

Instead of assuming that a PE/COFF section of type EFI_IMAGE_SCN_CNT_CODE
can always be mapped read-only, classify a section as a code section only
if it has the executable attribute set and the writable attribute cleared.

This adheres more closely to the PE/COFF spec, and avoids issues with
Linux OS loaders that may consist of a single read/write/execute section.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>