[Arm] Allow the use of arm_cde.h for C++

[Arm] Allow the use of arm_cde.h for C++

arm_cde.h includes the arm_mve_types.h header, which declares some C++
overloaded functions.

There is a superfluous `extern "C"` statement in arm_cde.h, which
encompasses these functions.  This means that if compiling for C++, the
overloaded functions are declared, but are declared without name
mangling.  Hence all the function names are the same and we have many
conflicting declarations.

Testing Done:
  Regression tested for arm-none-eabi.

gcc/ChangeLog:

2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/arm/arm_cde.h: Remove `extern "C"` when compiling for
	C++.

gcc/testsuite/ChangeLog:

2020-04-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* g++.target/arm/cde_mve.C: New test.