configure: Rewrite code for help message

configure: Rewrite code for help message

In the new form most lines of the code now look like the final output:
there is no leading echo command and the lines are shorter.

The resulting output is nearly identical: the only difference is a blank
character which was deliberately removed:

@@ -8,7 +8,7 @@
   --interp-prefix=PREFIX   where to find shared libraries, etc.
                            use %M for cpu name [/usr/gnemul/qemu-%M]
   --target-list=LIST       set target list (default: build everything)
-                           Available targets:  alpha-softmmu arm-softmmu
+                           Available targets: alpha-softmmu arm-softmmu
                            cris-softmmu i386-softmmu lm32-softmmu m68k-softmmu
                            microblaze-softmmu microblazeel-softmmu mips-softmmu
                            mips64-softmmu mips64el-softmmu mipsel-softmmu

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>