[clang] [Clang][Hexagon] Use --cstdlib flag to choose Picolibc (PR #183257)

[Carlos Seo via cfe-commits]

ceseo wrote:

This seems to be breaking the Windows on Arm bot (https://lab.llvm.org/buildbot/#/builders/124/builds/2152). Could you please take a look?

https://github.com/llvm/llvm-project/pull/183257