enable multilib by default in gcc 4.8, 4.9

Multilib is enabled by default in a normal gcc build, but the formula currently disables it by default. We should let it be enabled by default unless the user specifically requests to disable it. The multilibs take minimal amount of space and they enable gcc to build 32-bit programs, which is especially useful on OS X. For example, I used this to build Wine before it got full Clang support.

The patch applies the change to gcc 4.8 and 4.9 because those are the ones I've tested, but it should be extended to previous releases if someone has time to test them. Otherwise, I would suggest just changing 4.8 and 4.9 and letting people backport the change to older formulas if/when there's interest.