builder: refactor compiler-rt library

[aykevl]

This refactor makes adding a new library (such as a libc) much easier in the future as it avoids a lot of duplicate code. Additionally, CI should become a little bit faster (~15s) as build-builtins now uses the build cache.

Example for picolibc:

var picolibc = Library{
        name: "picolibc",
        cflags: func() []string {
                picolibcDir := filepath.Join(goenv.Get("TINYGOROOT"), "lib/picolibc/newlib/libc")
                return []string{"-Werror", "-Wall", "-std=c11", "-D_COMPILING_NEWLIB", "-D_POSIX_C_SOURCE=200809L", "-fshort-enums", "--sysroot=" + picolibcDir, "-I" + picolibcDir + "/tinystdio"}
        },
        sourceDir: "lib/picolibc/newlib/libc",
        sources: func(target string) []string {
                return picolibcSources
        },
}

[aykevl]

Replaced by #871.