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

This commit allows the ARM integrated assembler to parse
and assemble the code with .eabi_attribute, .cpu, and
.fpu directives.

To implement the feature, this commit moves the code from
AttrEmitter to ARMTargetStreamers, and several new test
cases related to cortex-m4, cortex-r5, and cortex-a15 are
added.

Besides, this commit also change the Subtarget->isFPOnlySP()
to Subtarget->hasD16() to match the usage of .fpu directive.

This commit changes the test cases:

* Several .eabi_attribute directives in
  2010-09-29-mc-asm-header-test.ll are removed because the .fpu
  directive already cover the functionality.

* In the Cortex-A15 test case, the value for
  Tag_Advanced_SIMD_arch has be changed from 1 to 2,
  which is more precise.