MdePkg: Fix broken coding style in Acpi64.h

MdePkg: Fix broken coding style in Acpi64.h

Bugzilla: 3516 (https://bugzilla.tianocore.org/show_bug.cgi?id=3516)

Fix a coding style issue raised by EccCheck. This issue (error code
7008) has been fixed by moving a nested union out of its structure.

Also add Acpi64.h to the "IgnoreFiles" list to stop the CI flagging
naming errors present prior to ACPI 64.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>