[mips] Handle the `long-calls` feature flags in the MIPS backend

[mips] Handle the `long-calls` feature flags in the MIPS backend

If the `long-calls` feature flags is enabled, disable use of the `jal`
instruction. Instead of that call a function by by first loading its
address into a register, and then using the contents of that register.

Differential revision: https://reviews.llvm.org/D35168

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308087 91177308-0d34-0410-b5e6-96231b3b80d8