debug: float and complex decoding functions for the Architecture type

These are analogous to the existing integer decoding functions.  We add
a field FloatByteOrder to Architecture for the endianness of floating
point values.

Also fixes some whitespace.

Change-Id: I2513affa81e072de9ab511327b1300c5767f1ead
Reviewed-on: https://go-review.googlesource.com/10248
Reviewed-by: Rob Pike <r@golang.org>