This patch is to support ongoing work for replacing "GCC/BFD" by "CLANG/LLD" on target PowerPC64 [1], by proposing a way to specify and/or locate a secondary ld.bfd linker.
This is necessary as LLD currently doesn't support PowerPC 32 bits, so we keep using BFD for the 32 bit stuff on PowePC64(LIB32 compatibility and STAND/slof/loader.)
- On native powerpc64 build, it adds parameter "-fuse-ld=bfd" to clang, so it locates default ld.bfd install.
- When cross compiling, it makes variable CROSS_BINUTILS_PREFIX mandatory, so it can locate the toolchain for the target platform, normally installed by pkg (as example, when ** CROSS_BINUTILS_PREFIX=powerpc64-unknown-freebsd12.0** is set, it looks for **powerpc64-unknown-freebsd12.0-ld.bfd**.
- In any case, variable LIB32LD can be set to manually specify the absolute path for an arbitrary linker and override the options above
- Default behavior for other platforms aren't changed, however these can make use of LIB32LD variable, optionally.
[1] https://wiki.freebsd.org/powerpc/llvm-elfv2