cmd/compile/internal/test: make TestIntendedInlining faster

cmd/compile/internal/test: make TestIntendedInlining faster

There is no need to build with -a. The go command should do the
right thing to pass the flags. Also, we only care packages
mentioned on the command line, so no need to add -gcflags=all=....

May fix #52081.

Change-Id: Idabcfe285c90ed5d25ea6d42abd7617078d3283a
Reviewed-on: https://go-review.googlesource.com/c/go/+/407015
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>