modp_b64: Fix GN build and add fuzzers.

modp_b64 is currently build with Makefile.
Fix the BUILD.gn so that it can be used and remove
Makefile usage.
Also add two fuzzers for encode and decode routines.

BUG=chromium:1012803
TEST=GN build work with modified ebuild.

Cq-Depend: chromium:1850664
Change-Id: I1756237d91630ea7e18e7c3ed055d2b3b532de0d