[asan] Remove zero FSR check on ARM.

The kernel on Nexus 5X returns error_code in ucontext which has
correct FSR_WRITE flag, but empty (zero) abort type field. Removing
the checks means that we will report all SEGVs as READ on very old
kernels, but will properly distinguish READ vs WRITE on moderately
old ones.

llvm-svn: 276803