EmbeddedPkg: introduce PlatformHasAcpiLib

Add the shorter-term library instance outlined in the previous patch to
EmbeddedPkg, so that we can imbue AcpiTableDxe with a protocol dependency
on EDKII_PLATFORM_HAS_ACPI_GUID.

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