toolchain: add glibc menu option for (custom) version

(E)GLIBC version numbers can be set in the menu and used through the variables BR2_TOOLCHAIN_BUILDROOT_GLIBC_VERSION and BR2_TOOLCHAIN_BUILDROOT_EGLIBC_VERSION
glibc: use glibc menu variables and support version 2.13

GLIBC_VERSION is set to BR2_TOOLCHAIN_BUILDROOT_GLIBC_VERSION or BR2_TOOLCHAIN_BUILDROOT_EGLIBC_VERSION for glibc and eglibc builds; a glibc 2.13 build is added for linux host builds
configs: support X86 host build for the qapitrace GUI

qapitrace runs (typically) remotely on a host; a linux x86_64 build has been (partially) verified on Debian Wheezy