Add `try` intrinsics

[liushuyu]

This pull request adds the try intrinsic, making it possible to handle panics.

A new option, -frust-panic is added to specify the panic handling strategy.

Note that currently, only try-catch mechanism is added in this pull request. To correctly handle the panic structures, an unwinder must also be implemented (in the runtime library).

[dkm]

You are also adding support for __builtin_expect