[ASan] Fix use of -asan-instrument-assembly in tests

Summary:
The option -asan-instrument-assembly is declared in the X86 backend.
If I test on PowerPC configured with LLVM_TARGETS_TO_BUILD=All then the
option is tolerated but ignored.
If I test on PowerPC configured with LLVM_TARGETS_TO_BUILD=PowerPC then
the testsuite fails with:
[ 93%] Generating ASAN_INST_TEST_OBJECTS.gtest-all.cc.powerpc64-inline.o
clang (LLVM option parsing): Unknown command line argument '-asan-instrument-assembly'.  Try: 'clang (LLVM option parsing) -help'

Fix this inconsistency by only adding the option if that toolchain was
built with the X86 backend included.

Reviewers: kcc, samsonov, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6190