[libc++abi] Add LIBCXXABI_ENABLE_PIC cmake option

This is on by default, since on many platforms and configurations
libc++abi.a gets statically linked into shared libraries and/or
PIE executables.

This change is a followup to https://reviews.llvm.org/D60005 which
allows us to default to PIC code, but disable this if needed (for
example on WebAssembly where PIC code its currently compatible with
static linking).

Differential Revision: https://reviews.llvm.org/D60049

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@357551 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed