[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>

src/Makefile

@@ -26,16 +26,16 @@ PRINTF		:= printf
 PERL		:= perl
 PYTHON		:= python
 TRUE		:= true
-CC		:= $(CROSS_COMPILE)gcc
-CPP		:= $(CC) -E
-AS		:= $(CROSS_COMPILE)as
-LD		:= $(CROSS_COMPILE)ld
-SIZE		:= $(CROSS_COMPILE)size
-AR		:= $(CROSS_COMPILE)ar
-RANLIB		:= $(CROSS_COMPILE)ranlib
-OBJCOPY		:= $(CROSS_COMPILE)objcopy
-NM		:= $(CROSS_COMPILE)nm
-OBJDUMP		:= $(CROSS_COMPILE)objdump
+CC		= $(CROSS_COMPILE)gcc
+CPP		= $(CC) -E
+AS		= $(CROSS_COMPILE)as
+LD		= $(CROSS_COMPILE)ld
+SIZE		= $(CROSS_COMPILE)size
+AR		= $(CROSS_COMPILE)ar
+RANLIB		= $(CROSS_COMPILE)ranlib
+OBJCOPY		= $(CROSS_COMPILE)objcopy
+NM		= $(CROSS_COMPILE)nm
+OBJDUMP		= $(CROSS_COMPILE)objdump
 OPENSSL		:= openssl
 CSPLIT		:= csplit
 PARSEROM	:= ./util/parserom.pl