[libomptarget][nfc] Add hook to easily disable building amdgcn bclib

[libomptarget][nfc] Add hook to easily disable building amdgcn bclib

This is useful when building LLVM with a toolchain that can't emit code
for amdgcn, e.g. because it overrides the include search path with headers
from another architecture, or the clang compiler is missing builtins.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D102229

Author: JonChesterfield

openmp/libomptarget/deviceRTLs/amdgcn/CMakeLists.txt

@@ -6,10 +6,18 @@
 #
 ##===----------------------------------------------------------------------===##
 #
-# Build the AMDGCN Device RTL if the ROCM tools are available
+# Build the AMDGCN Device RTL bitcode library using clang -ffreestanding
 #
 ##===----------------------------------------------------------------------===##
 
+set(LIBOMPTARGET_BUILD_AMDGCN_BCLIB TRUE CACHE BOOL
+  "Can be set to false to disable building this library.")
+
+if (NOT LIBOMPTARGET_BUILD_AMDGCN_BCLIB)
+  libomptarget_say("Not building AMDGCN device RTL: Disabled by LIBOMPTARGET_BUILD_AMDGCN_BCLIB")
+  return()
+endif()
+
 # Copied from nvptx CMakeLists
 if(CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "x86_64")
   set(aux_triple x86_64-unknown-linux-gnu)