[libc] change ASAN condition to generator expression

Previously, building LLVM-libc with GWP ASAN was conditioned on the flag
COMPILER_RT_BUILD_GWP_ASAN, which caused issues do to the default value
of the flag being set in the compiler-rt cmake, which is seperate. Now
GWP ASAN is included based on if it exists as a target, which is more
consistent.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D119789