Add tests to cc65 build as well as fileio test

[mlund]

Changes

• Fixed broken fileio.s assembly code for llvm-mos (see Tests for fileio and dirent #34) by changing calling convention.
• Fixed broken getrtc() function when running on Xemu.
• Add doxygen comments to selected functions
• Some type refactoring towards stdlib types.
• Add TARGET_EMULATION for detecting Xemu.
• New make test target for the cc65 Makefile
• New tests: test-fileio and test-time. Running looks something like this and passes on both cc65 and clang:

Running tests...
Test project /Users/mikael/github/mega65-libc/build
    Start 1: test-fileio
1/4 Test #1: test-fileio ......................   Passed    0.85 sec
    Start 2: test-integer-size
2/4 Test #2: test-integer-size ................   Passed    0.84 sec
    Start 3: test-memory
3/4 Test #3: test-memory ......................   Passed    0.84 sec
    Start 4: test-time
4/4 Test #4: test-time ........................   Passed    0.84 sec

100% tests passed, 0 tests failed out of 4

CC65 vs. Clang file size

Clang generated output is significantly smaller:

File	CC65	Clang
test-fileio.prg	3706	743
test-integer-size.prg	3152	135
test-memory.prg	3782	1203
test-time.prg	4822	1036

And apologies for this rather large PR; it should have been split into smaller pieces.

[lydon42]

Please elaborate on the all target order or change it back to be the top target.