[compiler-rt] Use -nostdinc++ in clang_rt.profile to avoid including C++ headers

Most of the code in compiler_rt is C code. However, clang_rt.profile
contains the InstrProfilingRuntime.cpp file, which builds as C++. This
means that including e.g. <stdint.h> will actually include libc++'s
<stdint.h> and then #include_next the system's <stdint.h>. However, if
the target we're building compiler-rt for isn't supported by libc++,
this will lead to a failure since libc++'s <stdint.h> includes <__config>,
which performs various checks.

Since the goal seems to *not* be including any header from the C++ Standard
Library in clang_rt.profile, using -nostdinc++ to ensure that doesn't
happen unknowingly seems to make sense.

rdar://65852694

Differential Revision: https://reviews.llvm.org/D84205