arch/power8: Rename to ppc64

POWER8 is a specific implementation of ppc64, which is by now outdated
(POWER9 has been on the market for a while). Rename arch/power8/ to
potentially cover a wider range of hardware.

TEST=Toolchains built before/after this commit can build coreboot for
     emulation/qemu-power8 from before/after this commit.

Change-Id: I2d6f08b12a9ffc8a652ddcd6f24ad85ecb33ca52
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/c/29943
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Timothy Pearson <tpearson@raptorengineering.com>

util/lint/check_lint_tests

@@ -6,7 +6,7 @@ UNDERSCORE='_'
 
 #lint-stable-000-license-headers
 TESTFILE000a=src/arch/x86/thread.c
-TESTFILE000b=src/arch/power8/misc.c
+TESTFILE000b=src/arch/ppc64/misc.c
 sed -i.bak 's/^[[:space:]]\*[[:space:]].*//' ${TESTFILE000a}
 sed -i.bak 's/^[[:space:]]\*[[:space:]]but WITHOUT ANY WARRANTY;//' ${TESTFILE000b}