GCC: Fix the toolchain LTO workaround for Python 3

[fkjagodzinski]

Summary of changes

Fix the GCC_ARM's check_if_obj_from_asm() to work properly with Python3.
Nightly tests failed with:

Traceback (most recent call last):
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/test_api.py", line 2091, in build_test_worker
    bin_file, _ = build_project(*args, **kwargs)
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/build_api.py", line 610, in build_project
    res = toolchain.link_program(resources, build_path, name)
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/toolchains/mbed_toolchain.py", line 778, in link_program
    self.link(elf, objects, libraries, lib_dirs, linker_script)
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/toolchains/gcc.py", line 326, in link
    asm_objects = self.get_asm_objects(objects)
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/toolchains/gcc.py", line 422, in get_asm_objects
    return [o for o in objects if self.check_if_obj_from_asm(o)]
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/toolchains/gcc.py", line 422, in <listcomp>
    return [o for o in objects if self.check_if_obj_from_asm(o)]
  File "/builds/ws/mbed-os-ci_build-GCC_ARM-nightly@4/mbed-os/tools/toolchains/gcc.py", line 415, in check_if_obj_from_asm
    match = self.DWARF_PRODUCER_RE.search(stdout.encode('utf-8'))
TypeError: cannot use a string pattern on a bytes-like object

In the Python3 version of the re module, the Unicode strings (str) and 8-bit strings (bytes) cannot be mixed.

Impact of changes

Migration actions required

Documentation

None

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@0xc0170, @adbridge, @jamesbeyond, @maciejbocianski

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[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 8 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-pytest

[0xc0170]

Known issue, client restarted