devel/freebsd-sysroot: Try to fix powerpc64_elfv1 flavor

The freebsd-gcc9 default ABI switch to elfv2 in
bf742277557cd64e9017df3844ca82173529c1ae breaks it:

/usr/local/bin/powerpc64-unknown-freebsd13.0-ld: /tmpccNlmC42.o: ABI version 2 is not compatible with ABI version 1 output
/usr/local/bin/powerpc64-unknown-freebsd13.0-ld: failed to merge target specific data of file /tmpccNlmC42.o