ARM stdenv: Update bootstrap tools

For some reason, the current bootstrap tools fail to build gettext:

init2.c:37: MPFR assertion failed: (64 - 0) == ((64 - 0)/8) * 8 && sizeof(mp_limb_t) == ((64 - 0)/8)
libxml/xpath.c: In function 'xmlXPathCompPathExpr':
libxml/xpath.c:10627:1: internal compiler error: Aborted
 xmlXPathCompPathExpr(xmlXPathParserContextPtr ctxt) {
 ^
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.
make[5]: *** [libxml/libxml_rpl_la-xpath.lo] Error 1

I didn't investigate why this is the case but rebuilding the bootstrap
tools seems to help.

I used this old-ish WIP branch https://github.com/dezgeg/nixpkgs/commits/arm-bootstrap
since latest master has even more problems with cross-compiling anything.
(I will eventually push this stuff and make the ARM bootstraps build on hydra.)