Fix asan_test.cc build on NetBSD

Summary:
Include <stdarg.h> for variable argument list macros (va_list, va_start etc).

Add fallback definition of _LIBCPP_GET_C_LOCALE, this is required for
GNU libstdc++ compatibility. Define new macro SANITIZER_GET_C_LOCALE.
This value is currently required for FreeBSD and NetBSD for printf_l(3) tests.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, vitalybuka, filcab, fjricci

Reviewed By: vitalybuka

Subscribers: llvm-commits, emaste, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36406

llvm-svn: 310323