test: support -ldflags for "rundir" tests, new -P option

For "rundir" tests, allow users to add in linker flags as well as
compiler flags, e.g.

// rundir -m -ldflags -w

The directive above will pass "-m" to the compiler on each package compilation
and "-w" to the linker for the final link.

In addition, if "-P" is specified with 'rundir', then for each compile
pass in "-p <X>" to set the packagepath explicitly, which is closer to
how the compiler is run by 'go build'.

Change-Id: I04720011a89d1bd8dcb4f2ccb4af1d74f6a01da1
Reviewed-on: https://go-review.googlesource.com/c/go/+/168977


Reviewed-by: Cherry Zhang <cherryyz@google.com>