fix build on arm64-Linux

[pedrolcl]

I have been requested in this PR to submit upstream the two patches already applied to the vcpkg port.

This PR includes two patches to the CMake build system:

• Define HAVE_CPU when the architecture is arm64 (not only for APPLE), and check CMAKE_CROSSCOMPILING if not.
• Add one library to pkgconfig

The solved issue was a build failure on arm64 (that you can see in the workflow CMake-Linux.yml added in PR #29 )

[ 12%] Building C object src/libmpg123/CMakeFiles/libmpg123.dir/home/runner/work/mpg123/mpg123/src/libmpg123/parse.c.o
In file included from /home/runner/work/mpg123/mpg123/src/libmpg123/mpg123lib_intern.h:307,
                 from /home/runner/work/mpg123/mpg123/src/libmpg123/parse.c:9:
/home/runner/work/mpg123/mpg123/src/libmpg123/optimize.h:198:2: error: #error "Bad decoder choice together with fixed point math!"
  198 | #error "Bad decoder choice together with fixed point math!"
      |  ^~~~~
gmake[2]: *** [src/libmpg123/CMakeFiles/libmpg123.dir/build.make:79: src/libmpg123/CMakeFiles/libmpg123.dir/home/runner/work/mpg123/mpg123/src/libmpg123/parse.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:498: src/libmpg123/CMakeFiles/libmpg123.dir/all] Error 2
gmake: *** [Makefile:136: all] Error 2
Error: Process completed with exit code 2.

[github-actions]

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