Revert r282483 - [cmake] Add linker option "-Wl,-z,defs" in standalone build

Revert r282483 as it causes build failures due to missing symbols when
not linking to -lgcc_s (i.e. doing pure LLVM stack build). The patch can
be reintroduced when the build system is fixed to add all needed
libraries (libunwind, compiler-rt).

llvm-svn: 282524