xcompile: Drop CC_RT_EXTRA_GCC for PPC64

It looks like some unused artifact:  The PPC64 Makefile.mk doesn't
pick it up. Also, the only other architecture using this (x86) has
linker flags there, not compiler flags.

Change-Id: I734542db9ee5b62d9a39d303d4092cd83dfef54b
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83577
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

util/xcompile/xcompile

@@ -415,7 +415,6 @@ arch_config_ppc64() {
 	TWIDTH="64"
 	TSUPP="ppc64"
 	TABI="linux-gnu" # there is no generic ABI on ppc64
-	CC_RT_EXTRA_GCC="-mcpu=power8 -mbig-endian"
 }
 
 # Right now, the clang reference toolchain is not building compiler-rt builtins