Ignore __fu<digits>__ symbols from COFF symbol tables

The debugger sometimes prints strange function names for given
addresses. For instance, with the following source code...

	4    procedure Foo is
	5       A : Integer;
	6    begin
	7       Do_Nothing (A'Address);
	8    end Foo;

... we can see...

    (gdb) info line 5
    Line 5 of "foo.adb" starts at address 0x4017ca <_ada_foo+6>
       and ends at 0x4017d2 <_fu29__system__scalar_values__is_is4+7>.
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

_fu29__system__scalar_values__is_is4 is an artificial symbol
generated by the linker, and interferes with the pc-to-symbol
resolution.  There isn't much in the general minimal_symbol
data that could help us identify them, so this patch changes
the COFF reader to simply ignore them.

gdb/ChangeLog:

	* coffread.c (is_import_fixup_symbol): New function.
	(record_minimal_symbol): Use is_import_fixup_symbol to
	detect import fixup symbols, and discard them.

gdb/testsuite/ChangeLog:

        * gdb.ada/win_fu_syms: New testcase.