eglibc builds for arm-eabi unconditionally enables the libc-do-syscall.S
file, which contains thumb assembly. It's unused in arm-eabi-nothumb case,
so just ifdef the actual code.
V2: use the patch from oe.dev.
The patch is submitted to sourceware bugzilla by Khem Raj back in October
of 2010, but the status is still UNCONFIRMED and the patch isn't commited.
