host-gcc: TOOLCHAIN_LIBS += libgcc (copied from compiler/gcc/)

Add missing -lgcc when compiling with ZEPHYR_TOOLCHAIN_VARIANT=host
merely copying some existing code from
'compiler/{clang,gcc}/target.cmake'.

This fixes compilation for the following boards with an x86
microprocessor:

 galileo, minnowboard, qemu_x86, qemu_x86_nommu, up_squared,
 up_squared_sbl

Compilation of the following boards with an X86_IAMCU microcontroller
still fail with a "cannot find -lgcc" error:

 arduino_101, qemu_x86_iamcu, quark_d2000_crb, quark_se_c1000_devboard,
 tinytile

This is _not_ a regression because these boards _already_ failed with
"undefined reference to __udivdi3" and other libgcc symbols.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

cmake/compiler/clang/target.cmake

@@ -33,6 +33,7 @@ foreach(isystem_include_dir ${NOSTDINC})
   list(APPEND isystem_include_flags -isystem ${isystem_include_dir})
 endforeach()
 
+# This libgcc code is partially duplicated in compiler/*/target.cmake
 execute_process(
   COMMAND ${CMAKE_C_COMPILER} ${TOOLCHAIN_C_FLAGS} --print-libgcc-file-name
   OUTPUT_VARIABLE LIBGCC_FILE_NAME

cmake/compiler/gcc/target.cmake

@@ -109,6 +109,7 @@ else()
   endif()
 
   if(NOT no_libgcc)
+    # This libgcc code is partially duplicated in compiler/*/target.cmake
     execute_process(
       COMMAND ${CMAKE_C_COMPILER} ${TOOLCHAIN_C_FLAGS} --print-libgcc-file-name
       OUTPUT_VARIABLE LIBGCC_FILE_NAME

cmake/compiler/host-gcc/target.cmake

@@ -27,3 +27,17 @@ else()
   endif()
 endif()
 find_program(CMAKE_CXX_COMPILER ${cplusplus_compiler}     CACHE INTERNAL " " FORCE)
+
+# This libgcc code is partially duplicated in compiler/*/target.cmake
+execute_process(
+  COMMAND ${CMAKE_C_COMPILER} ${CMAKE_C_FLAGS} --print-libgcc-file-name
+  OUTPUT_VARIABLE LIBGCC_FILE_NAME
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+  )
+assert_exists(LIBGCC_FILE_NAME)
+
+# While most x86_64 Linux distributions implement "multilib" and have
+# 32 bits libraries off the shelf, things like
+# "/usr/lib/gcc/x86_64-linux-gnu/7/IAMCU/libgcc.a" are unheard of.
+# So this does not support CONFIG_X86_IAMCU=y
+LIST(APPEND TOOLCHAIN_LIBS gcc)