tools/clang/lib/Driver/Tools.cpp code always passes x86 options to the front-end

[pawosm-arm]

Following options:

-def __x86_64 -def __x86_64__ -def __amd_64__amd64__ -def __k8 -def __k8__

are always passed to flang1 on every architecture, e.g. on ARM's AArch64, e.g.:

Target: aarch64-unknown-linux-gnu
Thread model: posix
InstalledDir: flang/bin
Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/5.4.0
Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6.0.0
Selected GCC installation: /usr/lib/gcc/aarch64-linux-gnu/5.4.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
flang1 demo.f90 -opt 0 -terse 1 -inform warn -nohpf -nostatic -y 129 2 -inform warn -x 19 0x400000 -quad -x 59 4 -x 15 2 -x 49 0x400004 -x 51 0x20 -x 57 0x4c -x 58 0x10000 -x 124 0x1000 -tp px -x 57 0xfb0000 -x 58 0x78031040 -x 47 0x08 -x 48 4608 -x 49 0x100 -stdinc flang/bin/../include:/usr/local/include:flang/lib/clang/4.0.1/include:/usr/include/aarch64-linux-gnu:/include:/usr/include -def unix -def __unix -def __unix__ -def linux -def __linux -def __linux__ -def __NO_MATH_INLINES -def __LP64__ -def __x86_64 -def __x86_64__ -def __LONG_MAX__=9223372036854775807L -def __SIZE_TYPE__=unsigned long int -def __PTRDIFF_TYPE__=long int -def __THROW= -def __extension__= -def __amd_64__amd64__ -def __k8 -def __k8__ -def __PGLLVM__ -freeform -vect 48 -y 54 1 -x 70 0x40000000 -y 163 0xc0000000 -x 189 0x10 -stbfile /tmp/demo-e55fd6.stb -modexport /tmp/demo-e55fd6.cmod -modindex /tmp/demo-e55fd6.cmdx -output /tmp/demo-e55fd6.ilm

[pawosm-arm]

Proposed approach to this issue: #47