Add find_c_sym and friends

Add find_c_sym and friends

for handling leading underscores when looking up symbols.
Necessary on MacOS, as there C symbols have a '_' prepended.
get_sym_addr (replacing get_elf_sym_addr) gets an argument to
specify if bare/raw/ELF symbols should be looked up or if decorated
C symbols should be looked up.  That reflects into tcc_get_symbol.
tcc_add_symbol is _not_ yet changed, but probably should be.