Fix build issue on NetBSD: Remove unneeded -nostdinc++ flag

Fix build issue on NetBSD: Remove unneeded -nostdinc++ flag

This flag isn't recognized by clang on NetBSD:
clang-3.9: warning: argument unused during compilation: '-nostdinc++'

$ pkg_info |grep -E 'lldb|llvm|clang'
llvm-3.9.0nb20160213 Low Level Virtual Machine compiler infrastructure
clang-3.9.0nb20160213 C language family frontend for LLVM
lldb-3.9.0nb20160213 next generation, high-performance debugger

$ uname -a
NetBSD chieftec 7.99.26 NetBSD 7.99.26 (GENERIC) dotnet/coreclr#0: Wed Feb 10 21:58:18 UTC 2016
root@chieftec:/tmp/netbsd-tmp/sys/arch/amd64/compile/GENERIC amd64

This flag was disabled also for Darwin in dotnet/coreclr#125 by @jkotas

Commit migrated from https://github.com/dotnet/coreclr/commit/47939a53d37033ea482e37222a06c71cceb030f0