package/clang: default linker to the cross GNU

By default clang is assuming the system linker /usr/bin/ld.  This
patchset updates the default to point at the cross toolchain ld.

Previously clang had been used on target only, this update is
required for host clang/clang++ to be used as a frontend for
cross-compiling.

Example build command:

output/host/bin/clang++ -mcpu=cortex-a53  \
  --sysroot /<abs path>/output/staging/ \
  -B /<abs path>/output/host/opt/ext-toolchain/ \
  foobar.cc -o foobar

Cc: Valentin Korenblit <valentinkorenblit@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>