[build] Allow for per-architecture cross-compilation prefixes

[build] Allow for per-architecture cross-compilation prefixes

We currently require the variable CROSS (or CROSS_COMPILE) to be set
to specify the global cross-compilation prefix.  This becomes
cumbersome when developing across multiple CPU architectures,
requiring frequent editing of build command lines and preventing
incompatible architectures from being built with a single command.

Allow a default cross-compilation prefix for each architecture to be
specified via the CROSS_COMPILE_<arch> variables.  These may then be
provided as environment variables, e.g. using

  export CROSS_COMPILE_arm32=arm-linux-gnu-
  export CROSS_COMPILE_arm64=aarch64-linux-gnu-
  export CROSS_COMPILE_loong64=loongarch64-linux-gnu-
  export CROSS_COMPILE_riscv32=riscv64-linux-gnu-
  export CROSS_COMPILE_riscv64=riscv64-linux-gnu-

This change requires some portions of the Makefile to be rearranged,
to allow for the fact that $(CROSS_COMPILE) may not have been set
until the build directory has been parsed to determine the CPU
architecture.

Signed-off-by: Michael Brown <mcb30@ipxe.org>