toolchain.mk: set SHELL = /bin/bash

while use cmd " make -f toolchain.mk -j2" for download gcc on ubuntu machine,
error occur
"/bin/sh: 1: Bad substitution make:*** [toolchain.mk:39:aarch64] Error 2"
root reason is that sh of ubuntu point to dash, not bash.
the code caused the error was
"(cd $(1)/bin && for f in *-none-linux*; do ln -s $$f $${f//-none} ; done;) \"

Signed-off-by: Yunli Liu <994605959@qq.com>
Acked-by: Jerome Forissier <jerome@forissier.org>