I have implemented the -ftest-coverage option. It works a bit different
from the gcc version. It output .tcov text file which looks almost the
same as a gcov file after a executable/so file is run.
Add lib/tcov.c file
Modify Makefiles to compile/install it
Add -ftest-coverage option in tcc.c/tcc.h/tcc-doc.texi
Add code to tccelf.c/tccgen.c/tccpe.c
Add gen_increment_tcov to tcc.h/*gen.c
unrelated changes:
Add sigemptyset in tccrun.c
Fix riscv64-gen.c tok_alloc label size
