[compiler-rt] Use try_compile_only to check for __ARM_FP

[mstorsjo]

Summary

This fixes detection when linking isn't supported (i.e. while building
builtins the first time).

Since 8368e4d54c459fe173d76277f17c632478e91add, after setting
CMAKE_TRY_COMPILE_TARGET_TYPE to STATIC_LIBRARY, this isn't strictly
needed, but is good for correctness anyway (and in case that commit
ends up reverted).

compiler-rt/lib/builtins/CMakeLists.txt

if (CAN_TARGET_${arch})
       # For ARM archs, exclude any VFP builtins if VFP is not supported
       if (${arch} MATCHES "^(arm|armhf|armv7|armv7s|armv7k|armv7m|armv7em)$")
         string(REPLACE ";" " " _TARGET_${arch}_CFLAGS "${TARGET_${arch}_CFLAGS}")
         check_compile_definition(__ARM_FP "${CMAKE_C_FLAGS} ${_TARGET_${arch}_CFLAGS}" COMPILER_RT_HAS_${arch}_VFP)
         if(NOT COMPILER_RT_HAS_${arch}_VFP)
           list(REMOVE_ITEM ${arch}_SOURCES ${arm_Thumb1_VFPv2_DP_SOURCES} ${arm_Thumb1_VFPv2_SP_SOURCES} ${arm_Thumb1_SjLj_EH_SOURCES})
         else()
           # Exclude any double-precision builtins if VFP is single-precision-only
-          check_c_source_compiles("#if !(__ARM_FP & 0x8)
+          try_compile_only(COMPILER_RT_HAS_${arch}_VFP_DP
+                           SOURCE "#if !(__ARM_FP & 0x8)
                                    #error No double-precision support!
                                    #endif
-                                   int main() { return 0; }" COMPILER_RT_HAS_${arch}_VFP_DP)
+                                   int main() { return 0; }")
           if(NOT COMPILER_RT_HAS_${arch}_VFP_DP)
             list(REMOVE_ITEM ${arch}_SOURCES ${arm_Thumb1_VFPv2_DP_SOURCES})
           endif()
         endif()
       endif()
 
       # Remove a generic C builtin when an arch-specific builtin is specified.
       filter_builtin_sources(${arch}_SOURCES ${arch})