[asan] Remove FreeBSD XFAIL from asan-sigbus.cpp test

[arichardson]

Summary

This test passes for me on FreeBSD 12.2.

[emaste]

See https://svnweb.freebsd.org/base?view=revision&revision=352807

Change the delivered signal for accesses to mapped area after the
backing object was truncated.  According to POSIX description for
mmap(2):
   The system shall always zero-fill any partial page at the end of an
   object. Further, the system shall never write out any modified
   portions of the last page of an object which are beyond its
   end. References within the address range starting at pa and
   continuing for len bytes to whole pages following the end of an
   object shall result in delivery of a SIGBUS signal.

   An implementation may generate SIGBUS signals when a reference
   would cause an error in the mapped object, such as out-of-space
   condition.

compiler-rt/test/asan/TestCases/Posix/asan-sigbus.cpp

 // Check handle_bus flag
 // Defaults to true
 // RUN: %clangxx_asan -std=c++11 %s -o %t
 // RUN: not %run %t 2>&1 | FileCheck %s -check-prefix=CHECK-BUS
 // RUN: %env_asan_opts=handle_sigbus=0 not --crash %run %t 2>&1 | FileCheck %s
 
 // UNSUPPORTED: ios
-// Instead of getting a SIGBUS error, we get a SIGSEGV
-// XFAIL: freebsd
 
 #include <assert.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <unistd.h>
 #include <string>