builtin, v: support -gc boehm on systems with libgc-dev installed

[spytheman]

This PR implements replacing malloc/calloc/realloc/free with GC_MALLOC/GC_MALLOC/GC_REALLOC/GC_FREE, when you pass -gc boehm during compilation, i.e. linking and using the Boehm-Demers-Weiser conservative C/C++ Garbage Collector from https://github.com/ivmai/bdwgc, which is often packaged and available on Linux distributions.

The intent is reducing the memory usage of programs, that process large amounts of text in batch mode on low memory environments (small VPSes), and for which a few ms of garbage collection pauses from time to time do not matter much.

NB1: this new option does not affect -autofree, it is independent from it, and can be used with or without -autofree (although I have not tested it with -autofree for now).

NB2: V still can not compile itself with -gc boehm, I am not sure yet why.
(edit: also vlib/builtin/map_test.v fails which may be related 🤔 )

fn main() {
        mut a := 'abc'.repeat(1000)
        for i in 0 .. 100000 {
        b := '$a|$a|$a|$a'
                a = a.replace('c', 'd')
                a = a.replace('d', 'x')
                a = a.replace('x', 'y')
                a = a.replace('y', 'z')
                a = a.replace('z', 'W')
                a = a.replace('z', 'c')
                if i % 10000 == 0 {
                        println('i: ${i:10} ${a[0..10]} ${b[30..60]}')
                }
        }
}