scripts: remove . from $PATH

Add CT_SanitizePath function which removes entries referring to ., /tmp
and non-existing directories from $PATH, and call it early in the
build script.

If . is in PATH, gcc-4.4.4 build breaks:

[ALL ] checking what assembler to use...
/tmp/build/targets/arm-unknown-linux-uclibcgnueabi/build/gcc-core-static/arm-unknown-linux-uclibcgnueabi/bin/as
...
[ALL ] config.status: creating as

i.e. "as" is supposed to be the arm-unknown-linux-uclibcgnueabi cross assembler,
but config.status creates a local "as" script which is calling the
host assembler.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[Yann E. MORIN: style fixes + explanations]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 20dd8cef1c8adff0aa3e78ae6d7acfbc45ed5a83)

 diff -durN gcc-3.4.6.orig/gcc/config/rs6000/linux-unwind.h gcc-3.4.6/gcc/config/rs6000/linux-unwind.h

 --- gcc-3.4.6.orig/gcc/config/rs6000/linux-unwind.h	2005-03-17 11:41:18.000000000 +0100

 +++ gcc-3.4.6/gcc/config/rs6000/linux-unwind.h	2007-08-15 23:07:00.000000000 +0200

 @@ -32,6 +32,7 @@

     these structs elsewhere;  Many fields are missing, particularly

     from the end of the structures.  */

 +#ifndef inhibit_libc

  struct gcc_vregs

  {

    __attribute__ ((vector_size (16))) int vr[32];

 @@ -320,3 +321,4 @@

  									\

      goto SUCCESS;							\

    } while (0)

 +#endif