missing ifdef around .thumb directive

[dann frazier]

When building the eglibc package w/ a toolchain that has thumb support disabled, I hit a FTBFS issue in ./ports/sysdeps/unix/sysv/linux/arm/eabi/libc-do-syscall.S due to an unguarded .thumb directive.

Following the lead of other files that specify .thumb, I added an ifdef around this:

#if defined(__thumb__)
        .thumb
        .syntax unified
#endif
        .hidden __libc_do_syscall

With that change, I was able to build/use eglibc. Obviously not a problem for any existing ubuntu binaries, but I would like to see a fix for this go upstream and into our source.