[libc] Fix libc build on NVPTX using wrong linker flag

Summary:
Ugly hacks abound, we can't actually test linker flags correctly
generically because not everyone has `nvlink` as a binary on their
machine which would then result in every single flag being unsupported.
This is the only 'linker flag' check we have, so just hard code it off.

Author: jhuber6

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -144,6 +144,14 @@ if( LLVM_ENABLE_ASSERTIONS )
   endif()
 endif()
 
+# If we are targeting a GPU architecture in a runtimes build we want to ignore
+# all the standard flag handling.
+if("${LLVM_RUNTIMES_TARGET}" MATCHES "^amdgcn" OR
+   "${LLVM_RUNTIMES_TARGET}" MATCHES "^nvptx64")
+  return()
+endif()
+
+
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)
   # When LLVM_ENABLE_EXPENSIVE_CHECKS is ON, LLVM will intercept errors
   # using assert(). An explicit check is performed here.
@@ -1141,7 +1149,7 @@ if (UNIX AND
 endif()
 
 # lld doesn't print colored diagnostics when invoked from Ninja
-if (UNIX AND CMAKE_GENERATOR MATCHES "Ninja")
+if (UNIX AND CMAKE_GENERATOR MATCHES "Ninja" AND NOT "${LLVM_RUNTIMES_TARGET}" MATCHES "^nvptx64")
   include(CheckLinkerFlag)
   check_linker_flag(CXX "-Wl,--color-diagnostics" LINKER_SUPPORTS_COLOR_DIAGNOSTICS)
   append_if(LINKER_SUPPORTS_COLOR_DIAGNOSTICS "-Wl,--color-diagnostics"