[libc] Fix pthread argument for scudo integration tests when using GCC

This adds "-pthreads" which appears to be a clang only
alias for "-pthread" (all the drivers check for both).

Use "-pthread" instead to be compatible with gcc.

Otherwise you get:
FAILED: bin/libc-gwp-asan-uaf-should-crash
: && /usr/bin/g++-11 <...> -pthreads <...> projects/libc/test/integration/scudo/liblibc_for_scudo_integration_test.a && :
g++-11: error: unrecognized command-line option ‘-pthreads’; did you mean ‘-pthread’?

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D143258