acpi, apei, ghes: Make NMI error notification to be GHES architecture extension.

Currently APEI depends on x86 architecture. It is because of NMI hardware
error notification of GHES which is currently supported by x86 only.
However, many other APEI features can be still used perfectly by other
architectures.

This commit adds two symbols:
1. HAVE_ACPI_APEI for those archs which support APEI.
2. HAVE_ACPI_APEI_NMI which is used for NMI code isolation in ghes.c
   file. NMI related data and functions are grouped so they can be wrapped
   inside one #ifdef section. Appropriate function stubs are provided for
   !NMI case.

Note there is no functional changes for x86 due to hard selected
HAVE_ACPI_APEI and HAVE_ACPI_APEI_NMI symbols.

Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Tony Luck <tony.luck@intel.com>