gdb/fortran: Add support for the ABS intrinsic function

gdb/fortran: Add support for the ABS intrinsic function

Adds support for the abs intrinsic function, this requires adding a
new pattern to the Fortran parser.  Currently only float and integer
argument types are supported to ABS, complex is still not supported,
this can be added later if needed.

gdb/ChangeLog:

	* f-exp.y: New token, UNOP_INTRINSIC.
	(exp): New pattern using UNOP_INTRINSIC token.
	(f77_keywords): Add 'abs' keyword.
	* f-lang.c: Add 'target-float.h' and 'math.h' includes.
	(value_from_host_double): New function.
	(evaluate_subexp_f): Support UNOP_ABS.

gdb/testsuite/ChangeLog:

	* gdb.fortran/intrinsics.exp: Extend to cover ABS.