gdb ignores DW_AT_name for C units when DW_AT_linkage_name is set

When a custom-language compiler emits DWARF with DW_LANG_C and gives subprograms/globals both a source DW_AT_name (e.g. 'clamp') and a mangled DW_AT_linkage_name (e.g. 'cheri.prog.clamp'), gdb uses the linkage name as the symbol's name: backtraces show the mangled name and 'print limit' fails with 'has unknown type'. Pass an empty linkageName to DIBuilder::createFunction/createGlobalVariableExpression; gdb then matches functions to ELF symbols by address and uses the source name. Also avoid parentheses in debugger-visible function names: gdb's linespec parses 'f(x)' as f with an argument list, so 'break f' gets a second location in a function named 'f(x)'. A runtime C main that calls the generated main also makes 'break main' hit two locations; renaming the C function and aliasing main to it (alias attribute on ELF) separates them.