dev-embedded/avr-libc: drop all keywords except amd64, bug #620316

dev-embedded/avr-libc: drop all keywords except amd64, bug #620316

dev-embedded/avr-libc is not installable as-is on any ARCH
gentoo currently supports. It should have been a KEYWORDS=avr
but there is no ARCH=avr profiles (at least yet).

To explain meaning of KEYWORDS let's look at a concrete example:
When cross-toolchain is installed with crossdev as 'crossdev -t avr'
on ia64 (as an example ARCH without existing KEYWORDS)
crossdev injects the following 'package.keywords':
    cross-avr/avr-libc * ~* -ia64 -~ia64

(as 'man 5 portage' says:)

    *  package is visible if it is stable on any architecture
    ~* package is visible if it is in testing on any architecture

Basically we need something (anything) in KEYWORDS
to act as a bearer of stable/unstable sign. Let it be amd64.

Bug: https://bugs.gentoo.org/620316
Package-Manager: Portage-2.3.13, Repoman-2.3.4