Remove libpac and v8 from the 64-bit blacklist.

external/chromium-libpac has been updated to depend on the copy of V8 in
external/chromium_org, and its makefiles updated to be multilib-safe.
external/v8 is thus unused and has been removed from the tree.

Bug: 12452053
Change-Id: I757fa187f7f67cd9a89d4b228d4ce3ed6d2858b1