bootstrap.sh: Make symlink sha*sum -> gsha*sum when using brew

Brew does only crate gsha*sum versions of this tool but the
contrib script of the prefix compiler makes use of this
tool without the g prefix.
1 job from buildsystem_improvements in 4 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #5153
img

04:08

 
Name Stage Failure
failed
img Test
make[2]: *** [all] Error 2
Makefile:848: recipe for target 'all' failed
make[2]: Leaving directory '/builds/athei/redox/prefix/binutils-build'
mk/prefix.mk:27: recipe for target '/builds/athei/redox/prefix/binutils-install' failed
make[1]: Leaving directory '/builds/athei/redox'
make[1]: *** [/builds/athei/redox/prefix/binutils-install] Error 2
Makefile:63: recipe for target 'ci-img' failed
make: *** [ci-img] Error 2
ERROR: Job failed: exit code 1