The most common extension for static libraries is .a and CMake generates
it with both GCC and Arm toolchains. Mbed CLI 1 generates .ar files with
the Arm Compiler, but since Mbed CLI 2 uses memap.py to generate map
files too, both extensions should be permitted.

This fixes a warning when building greentea tests:

    Malformed input found when parsing ARMCC map: _deps/greentea-client-build/libclient_userio.a(greentea_test_env.o)
    Malformed input found when parsing ARMCC map: _deps/greentea-client-build/libclient_userio.a(greentea_test_env.o)
    Malformed input found when parsing ARMCC map: _deps/greentea-client-build/libclient_userio.a(greentea_test_env.o)
    ...

which repeats over 30 times per test.