gdb: Move value_from_host_double into value.c and make more use of it

gdb: Move value_from_host_double into value.c and make more use of it

The function value_from_host_double can be moved from f-lang.c into
value.c as a generally useful function, and then used more widely.

Tested on X86-64/GNU Linux with no regressions.

gdb/ChangeLog:

	* f-lang.c (value_from_host_double): Moved to...
	* value.c (value_from_host_double): ...here.
	* value.h (value_from_host_double): Declare.
	* guile/scm-math.c (vlscm_convert_typed_number): Use
	value_from_host_double.
	(vlscm_convert_number): Likewise.
	* guile/scm-value.c (gdbscm_value_to_real): Likewise.
	* python/py-value.c (convert_value_from_python): Likewise.