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-4.3.2.orig/libgcc/config/sh/t-linux gcc-4.3.2/libgcc/config/sh/t-linux

--- gcc-4.3.2.orig/libgcc/config/sh/t-linux	2009-03-07 11:42:38.000000000 +0100

+++ gcc-4.3.2/libgcc/config/sh/t-linux	2009-03-07 11:03:39.000000000 +0100

@@ -15,7 +15,7 @@

 	-Wl,--soname=@shlib_base_name@.so.1 \

 	-Wl,--version-script=@shlib_map_file@ \

 	-o @multilib_dir@/@shlib_base_name@.so.1.tmp @multilib_flags@ \

-	@shlib_objs@ -lc && \

+	@shlib_objs@ && \

 	rm -f @multilib_dir@/@shlib_base_name@.so && \

 	if [ -f @multilib_dir@/@shlib_base_name@.so.1 ]; then \

 	  mv -f @multilib_dir@/@shlib_base_name@.so.1 \