ltrace: narrow down available architectures

First switch the architecture availability to positive logic since it's
cleaner and avoids issues when new ones are introduced.
Then look at the source for the available ones at sysdeps/linux-gnu/...

aarch64 -> NULL
arc -> NULL
arm -> hardcoded to little endian, so no armeb
avr32 -> NULL
blackfin -> NULL
microblaze -> NULL
mips -> little/big endian handled but not for 64 bits
nios2 -> NULL
ppc -> OK
sh -> NULL
sparc -> OK
x86 -> Both i386 and x86_64 handled
xtensa -> NULL

Fixes:
http://autobuild.buildroot.net/results/cd2/cd24e7b6f863ab413d76ca7a81bd357ddf1dc4f7/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>