[libcxx] Do not redeclare lgamma_r when targeting the LLVM C library

Summary:
We use `lgamma_r` for the random normal distrubtion support. In this
code we redeclare it, which caused issues with the LLVM C library as
this function has the `noexcept` flag. Also make sure we include
`features.h` when using the GPU so it gets this as well.

Author: jhuber6

libcxx/include/__configuration/platform.h

@@ -38,7 +38,9 @@
 #  else
 #    define _LIBCPP_GLIBC_PREREQ(a, b) 0
 #  endif // defined(__GLIBC_PREREQ)
-#endif   // defined(__linux__)
+#elif defined(__AMDGPU__) || defined(__NVPTX__)
+#  include <features.h>
+#endif
 
 #ifndef __BYTE_ORDER__
 #  error                                                                                                               \

libcxx/include/__random/binomial_distribution.h

@@ -97,7 +97,8 @@ class _LIBCPP_TEMPLATE_VIS binomial_distribution {
   }
 };
 
-#ifndef _LIBCPP_MSVCRT_LIKE
+// The LLVM C library provides this with conflicting `noexcept` attributes.
+#if !defined(_LIBCPP_MSVCRT_LIKE) && !defined(__LLVM_LIBC__)
 extern "C" double lgamma_r(double, int*);
 #endif