Remove _LIBCPP_EXTERN_TEMPLATE ARMCC compilation flag

[michalpasztamobica]

Summary of changes

The flag was added in #5993.

As per comment from @pan- , the flag is not necessary once libc++ is shipped with ARMCC.
According to Arm compiler 6.11 release notes:

Arm Compiler 6 includes the libc++ library as the C++ Standard Template Library.

The particular reason I am interested in removing the flag is that it gives compilation warnings if library functions are put to use (see #12300):

Warning: L6439W: Multiply defined Global Symbol std::__1::basic_streambuf<char, std::__1::char_traits<char>>::~basic_streambuf__sub_object() defined in .text._ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED2Ev(C:\Program Files\ARMCompiler6.13\bin\..\lib\libcxx\libcpp_w.l(ios.cpp.o)) rejected in favor of Symbol defined in .text._ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED2Ev(.main.o).
Warning: L6439W: Multiply defined Global Symbol std::__1::basic_istream<char, std::__1::char_traits<char>>::sentry::sentry__sub_object(std::__1::basic_istream<char, std::__1::char_traits<char>>&, bool) defined in .text._ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE6sentryC2ERS3_b(C:\Program Files\ARMCompiler6.13\bin\..\lib\libcxx\libcpp_w.l(ios.cpp.o)) rejected in favor of Symbol defined in .text._ZNSt3__113basic_istreamIcNS_11char_traitsIcEEE6sentryC2ERS3_b(.main.o).
Warning: L6439W: Multiply defined Global Symbol std::__1::basic_ostream<char, std::__1::char_traits<char>>::sentry::sentry__sub_object(std::__1::basic_ostream<char, std::__1::char_traits<char>>&) defined in .text._ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE6sentryC2ERS3_(C:\Program Files\ARMCompiler6.13\bin\..\lib\libcxx\libcpp_w.l(ios.cpp.o)) rejected in favor of Symbol defined in .text._ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE6sentryC2ERS3_(.main.o).
Warning: L6439W: Multiply defined Global Symbol std::__1::basic_ostream<char, std::__1::char_traits<char>>::sentry::~sentry__sub_object() defined in .text._ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE6sentryD2Ev(C:\Program Files\ARMCompiler6.13\bin\..\lib\libcxx\libcpp_w.l(ios.cpp.o)) rejected in favor of Symbol defined in .text._ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEE6sentryD2Ev(.main.o).
Warning: L6439W: Multiply defined Global Symbol std::__1::basic_streambuf<char, std::__1::char_traits<char>>::~basic_streambuf__deallocating() defined in .text._ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED0Ev(C:\Program Files\ARMCompiler6.13\bin\..\lib\libcxx\libcpp_w.l(ios.cpp.o)) rejected in favor of Symbol defined in .text._ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED0Ev(.main.o).

Impact of changes

Smaller image size, as the units definitions will not be copied into every compilation unit and we are linking against libc++ anyway.
Warnings about duplicate symbols, when libc++ is being used should go away.

Migration actions required

None

Documentation

None

---

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)

---

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

---

Reviewers

@ARMmbed/mbed-os-tools
@AnttiKauppila
@kjbracey-arm

---

[ciarmcom]

@michalpasztamobica, thank you for your changes.
@AnttiKauppila @kjbracey-arm @ARMmbed/mbed-os-tools @ARMmbed/mbed-os-maintainers please review.

[mark-edgeworth]

Not tested by me, but the code change looks good.

[adbridge]

@pan- @kjbracey-arm @AnttiKauppila could you please review ?

[kjbracey]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

I've fixed the version: Set to 6.0.0-alpha-2