compiler: move slice construction to callers of makeslice

compiler: move slice construction to callers of makeslice

    This is the gccgo version of https://golang.org/cl/141822:

        Only return a pointer p to the new slices backing array from makeslice.
        Makeslice callers then construct sliceheader{p, len, cap} explictly
        instead of makeslice returning the slice.

    This change caused the GCC backend to break the runtime/pprof test by
    merging together the identical functions allocateReflectTransient and
    allocateTransient2M.  This caused the traceback to be other than
    expected.  Fix that by making the functions not identical.

    This is a step toward updating libgo to the Go1.12beta1 release.

    Reviewed-on: https://go-review.googlesource.com/c/155937

From-SVN: r267660