[otbn] Allow the user to supply a linker script with -T in otbn-ld
[otbn] Allow the user to supply a linker script with -T in otbn-ld

It turns out that binutils ld accumulates -T options, so otbn-ld
didn't really support something like

   path/to/otbn-ld -T my-linker-script.ld my-object.o

since it expanded to something like

   riscv32-unknown-elf-ld -T default-linker-script.ld \
                          -T my-linker-script.ld \
                          my_object.o

This latter command isn't what you want, because *it* is equivalent to
something like

   cat default-linker-script.ld my-linker-script.ld >combined.ld
   riscv32-unknown-elf-ld -T combined.ld my_object.o

which is no good if you want to override, rather than augment, the
default linker script.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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