gcc: or1k: use 20170218 tag instead of branch

As pointed out by Joel Stanley:
https://patchwork.ozlabs.org/patch/863011/



Github now longer provides the exact same tarball for the or1k musl-5.4.0
tarball, breaking the build.  Even more, musl-5.4.0 is the name of a git
branch, not a tag.

Fix both problems by changing to the or1k-musl-5.4.0-20170218 tag, which
points to the exact same git commit.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>