math/big: add %x float format

big.Float already had %p for printing hex format,
but that format normalizes differently from fmt's %x
and ignores precision entirely.

This CL adds %x to big.Float, matching fmt's behavior:
the verb is spelled 'x' not 'p', the mantissa is normalized
to [1, 2), and precision is respected.

See golang.org/design/19308-number-literals for background.

For #29008.

Change-Id: I9c1b9612107094856797e5b0b584c556c1914895
Reviewed-on: https://go-review.googlesource.com/c/160249


Reviewed-by: Robert Griesemer <gri@golang.org>