libacpi: fix arm64 build

libacpi: fix arm64 build

The arm64 build for libacpi was broken due to two reasons:

1. ACPI_BUILD_DIR was appended twice to dsdt_anycpu_arm.c.
2. The inclusion of firmware/Rules.mk overrided XEN_TARGET_ARCH, which
   made CONFIG_ARM disappear.

Fix those by:

1. Correctly generate full path for dsdt_anaycpu_arm.c.
2. Include tools/Rules.mk instead, because libacpi/Makefile doesn't rely
   on settings in firmware/Rules.mk.

While at it, use CONFIG_ARM_64 instead of CONFIG_ARM as it is more
accurate.

Reported-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>