[ARM] Implement eabi_attribute, cpu, and fpu directives.

Summary:
This commit adds the eabi_attribute, cpu, and fpu directives
support to the ARM assembly parser.  Besides, this commit
also moves switchVendor(), emitAttribute(),
emitTextAttribute(), emitFPU() to ARMTargetStreamer.

It has been verified that the output binaries of the changed
test case remain unchanged.

CC: llvm-commits, aemerson

Differential Revision: http://llvm-reviews.chandlerc.com/D1987
[arm] Allow multiple fpu directive.

If there are two .fpu directives, then the later one will
overwrite the former one.