[libc] Don't try to use MPFR with the GPU build for now

Summary:
We don't have the infastructure to support MPFR on the GPU. We should
disable this categorically on GPU builds for now.

Author: jhuber6

libc/cmake/modules/LLVMLibCCheckMPFR.cmake

@@ -2,6 +2,8 @@ set(LLVM_LIBC_MPFR_INSTALL_PATH "" CACHE PATH "Path to where MPFR is installed (
 
 if(LLVM_LIBC_MPFR_INSTALL_PATH)
   set(LIBC_TESTS_CAN_USE_MPFR TRUE)
+elseif(LIBC_TARGET_ARCHITECTURE_IS_GPU)
+  set(LIBC_TESTS_CAN_USE_MPFR FALSE)
 else()
   try_compile(
     LIBC_TESTS_CAN_USE_MPFR