Align natural-format register values to the same column

Currently, commands such as "info reg", "info all-reg", as well as register
window in the TUI print badly aligned columns, like here:

eax            0x1      1
ecx            0xffffd3e0       -11296
edx            0xffffd404       -11260
ebx            0xf7fa5ff4       -134586380
esp            0xffffd390       0xffffd390
ebp            0xffffd3c8       0xffffd3c8
esi            0x0      0
edi            0x0      0
eip            0x8048b60        0x8048b60 <main+16>
eflags         0x286    [ PF SF IF ]
cs             0x23     35
ss             0x2b     43
ds             0x2b     43
es             0x2b     43
fs             0x0      0
gs             0x63     99

After this patch, these commands print the third column values consistently
aligned one under another, provided the second column is not too long.
Originally, the third column was (attempted to be) aligned using a simple tab
character. This patch changes the alignment to spaces only. The tests checking
the output and expecting the single tab have been fixed in a previous patch, so
this change doesn't break any.

gdb/ChangeLog:

	* infcmd.c (default_print_one_register_info): Align natural-format
	column values consistently one under another.
	(pad_to_column): New function.