Host as flags.2

These changes replace ASM_DRIVER_WORKAROUND by a new configure variable HOST_AS_FLAGS. This variable decides how to call the assembler (when called via clang); apparently, this differs depending on the architecture, and is also different from the flags necessary to call clang to compile, to call llc, or to call ld.

I tested this on ARMv6, and it should also work fine on ARMv7. I also tested this on x86-64 on OS X.