[Clang][Hexagon] Use --cstdlib flag to choose Picolibc (#183257)
Update the Hexagon toolchain to use --cstdlib flag to allow users to
choose Picolibc as the C library.
This allows proper selection of C library when targeting
Hexagon environments.
depends on https://github.com/llvm/llvm-project/pull/183254
Signed-off-by: Kushal Pal <kushpal@qti.qualcomm.com>