debug/ltrace: fix building on x86

ltrace uses i386 and x86_64, whereas crosstool-NG use x86 for both cases.
Fix that by detecting what bitness we're building for, and pass appropriate
i386 or x86_64 to ltrace's configure.

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."