[Coverage] Disable Linux/coverage-missing.cc in AddressSanitizer-i386-linux testsuite

sancov.py relies on objdump -d to obtain the number of instrumented PCs.
The i386 %dynamiclib will now include .plt entries that are not recognized by objdump,

"sancov.py: found 0 instrumented PCs in *.so",

causing AddressSanitizer-i386-linux to fail.

Change it back to x86-target-arch after %sancov switches to a more robust approach.

llvm-svn: 343178