[cmake] Disable a GCC optimization when building LLVM for MIPS

GCC when compiling LLVM for MIPS can introduce a jump to an uninitialized
value when shrink wrapping is enabled. As shrink wrapping is enabled in
GCC at all optimization levels, it must be disabled. This bug exists for
all versions of GCC since 4.9.2.

This partially resolves PR37701 / GCC PR target/86069.

Patch by Simon Dardis.

Differential Revision: https://reviews.llvm.org/D48069

llvm-svn: 359376