Fix PowerPC-targeted cross-builds performed on 64-bit platforms.

Fix PowerPC-targeted cross-builds performed on 64-bit platforms.
Only PowerPC supports both 32-bit and 64-bit targets and the
BFD_DEFAULT_TARGET_SIZE is used by the binutils code to reflect
the preferred ABI. We define BFD_DEFAULT_TARGET_SIZE for all
platforms, but based on the build machine. As such 64-bit build
machines defined BFD_DEFAULT_TARGET_SIZE incorrectly for 32-bit
targets, but since this only affects PowerPC it went unnoticed
for a long time.
The fix is to define BFD_DEFAULT_TARGET_SIZE based on the target
architecture.

PR: amd64/102996
MFC after: 1 month