[AArch64] Enable CLI for Armv8.6-A f64mm

[AArch64] Enable CLI for Armv8.6-A f64mm

This patch is necessary for sve-ld1ro intrinsic I posted in
https://gcc.gnu.org/ml/gcc-patches/2020-01/msg00466.html .

I had mistakenly thought this option was already enabled upstream.

This provides the option +f64mm, that turns on the 64 bit floating point
matrix multiply extension.  This extension is only available for
AArch64.  Turning on this extension also turns on the SVE extension.

This extension is optional and only available at Armv8.2-A and onward.

We also add the ACLE defined macro for this extension.

gcc/ChangeLog:

2020-01-17  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/aarch64/aarch64-c.c (_ARM_FEATURE_MATMUL_FLOAT64):
	Introduce this ACLE specified predefined macro.
	* config/aarch64/aarch64-option-extensions.def (f64mm): New.
	(fp): Disabling this disables f64mm.
	(simd): Disabling this disables f64mm.
	(fp16): Disabling this disables f64mm.
	(sve): Disabling this disables f64mm.
	* config/aarch64/aarch64.h (AARCH64_FL_F64MM): New.
	(AARCH64_ISA_F64MM): New.
	(TARGET_F64MM): New.
	* doc/invoke.texi (f64mm): Document new option.

gcc/testsuite/ChangeLog:

2020-01-17  Matthew Malcomson  <matthew.malcomson@arm.com>

	* gcc.target/aarch64/pragma_cpp_predefs_2.c: Check for f64mm
	predef.