Stack alignment + some random fixes

There are some fixes I made while I was trying to use libconcurrent. I didn't want to create a pull request for each one, you can cherry-pick specific commits if you don't like all of these.

cbdb5b0 removed executable bit from non executable files
8efb89c this one took me a while to figure out. Some glibc/libstdc++ functions use xmm registers on x86_64, but it will segfault if it tries to push/save an xmm register to the stack if it's only 8 byte aligned. They're 128bit registers, so they need 16 byte alignment.
1fcf974 added missing `.gitignore`
f21f9f3 stripping the static library gives `libconcurrent.a: error adding symbols: Archive has no index; run ranlib to add one` when I try to later link it with a command.