libc/glibc: fix building for seemingly native toolchains

Build glibc with -O2 as a fix/workaround to building
seemingly-native toolchains.

See:
- docs/overview.txt
- docs/known-issues.txt
- http://sourceware.org/ml/crossgcc/2009-09/msg00055.html

 # x86 specific options

 config ARCH_x86

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         http://www.intel.com/