guix: Fix powerpc64(le) dynamic linker name

I used Guix's values for the powerpc64(le) dynamic linkers, and the
/lib-prefix seems to be a Guix-ism rather than standard. The standard
path for the linker-loaders start with /lib64.

I've taken the new loader values from SYSDEP_KNOWN_INTERPRETER_NAMES in
glibc's sysdeps/unix/sysv/linux/powerpc/ldconfig.h file.

For future reference, loader path values can also be found on glibc's
website: https://sourceware.org/glibc/wiki/ABIList?action=recall&rev=16

Author: dongcarl

contrib/guix/libexec/build.sh

@@ -169,8 +169,8 @@ case "$HOST" in
                 arm-linux-gnueabihf)   echo /lib/ld-linux-armhf.so.3 ;;
                 aarch64-linux-gnu)     echo /lib/ld-linux-aarch64.so.1 ;;
                 riscv64-linux-gnu)     echo /lib/ld-linux-riscv64-lp64d.so.1 ;;
-                powerpc64-linux-gnu)   echo /lib/ld64.so.1;;
-                powerpc64le-linux-gnu) echo /lib/ld64.so.2;;
+                powerpc64-linux-gnu)   echo /lib64/ld64.so.1;;
+                powerpc64le-linux-gnu) echo /lib64/ld64.so.2;;
                 *)                     exit 1 ;;
             esac
         )