Update to the GCC 5.5 release. This brings more than 250 final bug
fixes compared to GCC 5.4 and is the last release planned on the GCC 5
branch which is now closed.

files/patch-aarch64-support, files/patch-disable-armvhf-config.gcc,
files/patch-libgcc-config-arm-unwind-arm, and files/patch-x86-64-fix-m16
all have been merged upstream in between GCC 5.4 and 5.5 and can thus
be removed; the same is the case for most of files/patch-libc++.

Finally, the tarball is now compressed using xz instead of bzip2.

PR: 216266