* gdbarch.sh: Make the `default' field really default to zero, as

* gdbarch.sh: Make the `default' field really default to zero, as
documented.

Bring COERCE_FLOAT_TO_DOUBLE under gdbarch's control.
* valops.c (COERCE_FLOAT_TO_DOUBLE): Rework definition to be
more function-like.
(default_coerce_float_to_double, standard_coerce_float_to_double):
New functions.
(value_arg_coerce): Adjust for new definition.
* value.h (default_coerce_float_to_double,
standard_coerce_float_to_double): New declarations for the above.
* gdbarch.sh (coerce_float_to_double): New entry, replacing macro.
* gdbarch.c, gdbarch.h: Regenerated.
* tm-alpha.h, tm-fr30.h, tm-m32r.h, tm-mips.h, tm-hppa.h,
tm-rs6000.h, tm-sh.h, tm-sparc.h (COERCE_FLOAT_TO_DOUBLE): Change
definitions.
* mips-tdep.c (mips_coerce_float_to_double): Supply our own custom
function here.
(mips_gdbarch_init): Install that as our coerce_float_to_double
function.