aarch64: Add front-end argument type checking for target builtins

aarch64: Add front-end argument type checking for target builtins

In implementing the ACLE read/write system register builtins it was
observed that leaving argument type checking to be done at expand-time
meant that poorly-formed function calls were being "fixed" by certain
optimization passes, meaning bad code wasn't being properly picked up
in checking.

Example:

  const char *regname = "amcgcr_el0";
  long long a = __builtin_aarch64_rsr64 (regname);

is reduced by the ccp1 pass to

  long long a = __builtin_aarch64_rsr64 ("amcgcr_el0");

As these functions require an argument of STRING_CST type, there needs
to be a check carried out by the front-end capable of picking this up.

The introduced `check_general_builtin_call' function will be called by
the TARGET_CHECK_BUILTIN_CALL hook whenever a call to a builtin
belonging to the AARCH64_BUILTIN_GENERAL category is encountered,
carrying out any appropriate checks associated with a particular
builtin function code.

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.cc (aarch64_general_check_builtin_call):
	New.
	* config/aarch64/aarch64-c.cc (aarch64_check_builtin_call):
	Add `aarch64_general_check_builtin_call' call.
	* config/aarch64/aarch64-protos.h (aarch64_general_check_builtin_call):
	New.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/acle/rwsr-3.c: New.