Enhancement to dmd's -vasm switch

[Walter Bright]

https://github.com/dlang/dmd/pull/24143

It makes it trivial to dump the generated code for a specific function from a 
large source code base. Just name the function, and voila! Before I'd have to 
extract all the .o files from the library, and then use grep to figure out which 
one had the desired function in it, and then obj2asm the function. bazinga!