make.bash: fix misuse of continue

Apparently, in bash, the "continue" keyword can only be used
inside of a loop, not in an if block. If readelf exists but $CC
does not, make.bash emits a warning:

./make.bash: line 135: continue: only meaningful in a `for', `while', or `until' loop

Change it to a conditional.

Change-Id: I00a0940ed99bc0c565094e506705961b6b3d362e
Reviewed-on: https://go-review.googlesource.com/c/go/+/320170


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>