examples: make: specify CROSS_COMPILE for fomu

When BOARD=fomu, use the riscv cross-compiler.  Otherwise, use the
default arm compiler.  This can be overridden by passing
CROSS_COMIPLE on the command line.

Note that there are now three common risc-v prefixes:

    - riscv32-unknown-elf- : Common for users who compile their own
    - riscv64-unknown-elf- : Upstream multiarch toolchain from SiFive
    - riscv-none-embed-    : xPack embedded version of SiFive toolchain

Here we assume users are using the `riscv-none-embed-` toolchain from
xPack, because it appears to be growing more common.  Additionally,
there is much confusion surrounding `riscv64-unknown-elf-`, which
actually includes both 32- and 64-bit runtimes and can generate software
for both.

Signed-off-by: Sean Cross <[email protected]>

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