Mark a bash script with a /bin/bash shebang

This fixes an ab/aosp-master-ndk build failure on win32 and win64. I think
it's only used on Windows, because Linux and Darwin already have
Lib/plat-{darwin,linux2} directories. Windows doesn't have one, so the
build system copies plat-generic to plat-win32, then runs regen.

${CCINSTALL//\\//} is a bashish

Demo:

$ export FOO=a-b-c-d
$ bash -c 'echo ${FOO/-/+}'
a+b-c-d
$ bash -c 'echo ${FOO//-/+}'
a+b+c+d
$ dash -c 'echo ${FOO/-/+}'
dash: 1: Bad substitution
$ dash -c 'echo ${FOO//-/+}'
dash: 1: Bad substitution

I think I could use 'tr' instead, but this is less likely to break
something.

Test: ./python/build.py --host windows
Change-Id: Ib3565d63ca2fa12724c88aed5b84fd352c02f3d8