libc/glibc: do not overwrite existing bits/syscall.h

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Rafael C <groups.r2@gmail.com>
Cc: Jérôme BARDON <bardon.pro@gmail.com>
Cc: Daniel Price <daniel.price@gmail.com>

 # Alpha specific configuration file

 ## select ARCH_SUPPORTS_32

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_WITH_CPU

 ## select ARCH_SUPPORTS_WITH_TUNE

 ##

 ## help The Alpha architecture.