Tools: Add a workaround for the GCC_ARM & LTO bug

This is a workaround for the GCC not using the strong symbols from
C files to override the weak symbols from ASM files. This GCC bug is only
present when building with the link-time optimizer (LTO) enabled. For
more details please see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83967

This can be fixed by changing the order of object files in the linker
command; objects providing the weak symbols and compiled from assembly
must be listed before the objects providing the strong symbols.
To keep things simple, ALL object files from ASM are listed before
other object files.

tools/toolchains/gcc.py

@@ -17,7 +17,7 @@ limitations under the License.
 import re
 import fnmatch
 from os.path import join, basename, splitext, dirname, exists
-from os import getenv
+from os import getcwd, getenv
 from distutils.spawn import find_executable
 from distutils.version import LooseVersion
 
@@ -35,6 +35,7 @@ class GCC(mbedToolchain):
 
     GCC_RANGE = (LooseVersion("9.0.0"), LooseVersion("10.0.0"))
     GCC_VERSION_RE = re.compile(b"\d+\.\d+\.\d+")
+    DWARF_PRODUCER_RE = re.compile(r'(DW_AT_producer)(.*:\s*)(?P<producer>.*)')
 
     def __init__(self, target,  notify=None, macros=None, build_profile=None,
                  build_dir=None, coverage_patterns=None):
@@ -156,6 +157,7 @@ class GCC(mbedToolchain):
 
         self.ar = join(tool_path, "arm-none-eabi-ar")
         self.elf2bin = join(tool_path, "arm-none-eabi-objcopy")
+        self.objdump = join(tool_path, "arm-none-eabi-objdump")
 
         self.use_distcc = (bool(getenv("DISTCC_POTENTIAL_HOSTS", False))
                            and not getenv("MBED_DISABLE_DISTCC", False))
@@ -306,12 +308,31 @@ class GCC(mbedToolchain):
             self.default_cmd(cmd)
             mem_map = preproc_output
 
+        # NOTE: GCC_ARM_LTO_WORKAROUND
+        # This is a workaround for the GCC not using the strong symbols from
+        # C files to override the weak symbols from ASM files. This GCC bug is only
+        # present when building with the link-time optimizer (LTO) enabled. For
+        # more details please see:
+        #   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83967
+        #
+        # This can be fixed by changing the order of object files in the linker
+        # command; objects providing the weak symbols and compiled from assembly
+        # must be listed before the objects providing the strong symbols.
+        # To keep things simple, ALL object files from ASM are listed before
+        # other object files.
+        asm_objects = []
+        if '-flto' in self.ld:
+            asm_objects = self.get_asm_objects(objects)
+        reorg_objects = (
+            [o for o in objects if o in asm_objects] +
+            [o for o in objects if o not in asm_objects]
+        )
         # Build linker command
         map_file = splitext(output)[0] + ".map"
         cmd = (
             (self.coverage_ld if self.coverage_patterns else self.ld) +
             ["-o", output, "-Wl,-Map=%s" % map_file] +
-            objects +
+            reorg_objects +
             ["-Wl,--start-group"] +
             libs +
             ["-Wl,--end-group"]
@@ -383,6 +404,21 @@ class GCC(mbedToolchain):
             exec_name = join(TOOLCHAIN_PATHS['GCC_ARM'], 'arm-none-eabi-gcc')
             return exists(exec_name) or exists(exec_name + '.exe')
 
+    def check_if_obj_from_asm(self, obj_file):
+        """Check if obj_file was build by the GNU Assembler."""
+        dw_producer = ''
+        cmd = [self.objdump, '--dwarf=info', obj_file]
+        stdout, stderr, rc = run_cmd(cmd, work_dir=getcwd(), chroot=self.CHROOT)
+        if rc != 0:
+            return False
+        match = self.DWARF_PRODUCER_RE.search(stdout.encode('utf-8'))
+        if match:
+            dw_producer = match.group('producer')
+        return 'GNU AS' in dw_producer
+
+    def get_asm_objects(self, objects):
+        """Return a list of object files built from ASM."""
+        return [o for o in objects if self.check_if_obj_from_asm(o)]
 
 class GCC_ARM(GCC):
     pass