Log Message:
Add GCC_TARGET_MACHINE, defaulting to MACHINE_GNU_ARCH, and use it in
preference to target_noncanonical so that the user can override if
required, e.g. in a multilib environment where target_noncanonical will
change based on current ABI.

Additionally, ensure that it comes first in the RPATH so that when
using USE_PKGSRC_GCC_RUNTIME with in-pkgsrc gcc we pick up the correct
libraries.