[cmake] Make check_linker_flags operate via linker flags

[cmake] Make check_linker_flags operate via linker flags

`check_linker_flags` currently sets the *compiler* flags (via
`CMAKE_REQUIRED_FLAGS`), and thus implicitly relies on cmake's default
behavior of passing the compiler flags to the linker. This breaks when
cmake's build rules have been altered to not pollute the link line with
compiler flags (which can be desirable for build cleanliness). Instead,
set `CMAKE_EXE_LINKER_FLAGS` explicitly and use `CMP0056` to ensure the
linker flags are passed along. Additionally, since we're inside a
function, we can just alter the variable directly (as the alteration
will be limited to the scope of the function) rather than saving and
restoring the old value.

Differential Revision: https://reviews.llvm.org/D39431

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316972 91177308-0d34-0410-b5e6-96231b3b80d8