This patch to the Go frontend moves slice construction to callers of
makeslice.  This is the gccgo version of https://golang.org/cl/141822,
which was described as:

    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.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian