compiler-rt: Fix cmake check for _Float16 and __bf16

When using check_c_source_compiles cmake function, main has to be
provided, otherwise while compile step will be fine but linking step
will fail as there will be no main function and test will fail and
disable building HF builtins into compiler-rt

Fixes configure errors like below

aarch64-yoe-linux-ld.lld: error: undefined symbol: main
 referenced by start.S:81 (../sysdeps/aarch64/start.S:81)
               /mnt/b/yoe/master/build/tmp/work/cortexa72-cortexa53-crypto-yoe-linux/compiler-rt/18.1.0/recipe-sysroot/usr/lib/Scrt1.o:(_start)
 referenced by start.S:82 (../sysdeps/aarch64/start.S:82)
               /mnt/b/yoe/master/build/tmp/work/cortexa72-cortexa53-crypto-yoe-linux/compiler-rt/18.1.0/recipe-sysroot/usr/lib/Scrt1.o:(_start)
 aarch64-yoe-linux-clang: error: linker command failed with exit code 1 (use -v to see invocation)

Author: kraj

compiler-rt/lib/builtins/CMakeLists.txt

@@ -828,10 +828,10 @@ else ()
           endif()
         endif()
       endif()
-      check_c_source_compiles("_Float16 foo(_Float16 x) { return x; }"
+      check_c_source_compiles("_Float16 foo(_Float16 x) { return x; } int main(void) { return foo(1.0); }"
                               COMPILER_RT_HAS_${arch}_FLOAT16)
       append_list_if(COMPILER_RT_HAS_${arch}_FLOAT16 -DCOMPILER_RT_HAS_FLOAT16 BUILTIN_CFLAGS_${arch})
-      check_c_source_compiles("__bf16 foo(__bf16 x) { return x; }"
+      check_c_source_compiles("__bf16 foo(__bf16 x) { return x; } int main(void) { return foo(1.0); }"
                               COMPILER_RT_HAS_${arch}_BFLOAT16)
       # Build BF16 files only when "__bf16" is available.
       if(COMPILER_RT_HAS_${arch}_BFLOAT16)