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POWER8 is a specific implementation of ppc64, which is by now outdated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/xcompile/xcompile

@@ -303,7 +303,7 @@ EOF
 }
 
 # Architecture definitions
-SUPPORTED_ARCHITECTURES="arm arm64 mipsel riscv x64 x86 power8"
+SUPPORTED_ARCHITECTURES="arm arm64 mipsel riscv x64 x86 ppc64"
 
 # TARCH: local name for the architecture
 # (used as CC_${TARCH} in the build system)
@@ -366,12 +366,12 @@ arch_config_mipsel() {
 	TENDIAN="EL"
 }
 
-arch_config_power8() {
-	TARCH="power8"
+arch_config_ppc64() {
+	TARCH="ppc64"
 	TBFDARCHS="powerpc"
 	TCLIST="powerpc64"
 	TWIDTH="64"
-	TSUPP="power8"
+	TSUPP="ppc64"
 	TABI="linux-gnu" # there is no generic ABI on ppc64
 	CC_RT_EXTRA_GCC="-mcpu=power8 -mbig-endian"
 }