Fix building arm EABI

Fix building arm EABI

  The syscall() impl on aeabi comes from syscall-eabi.S thus we do not
  need the generic syscall() impl.

  Fixes:
    AS libc/sysdeps/linux/arm/syscall-eabi.os
    CC libc/sysdeps/linux/common/syscall.os
libc/libc_so.a(syscall.os): In function `syscall':
syscall.c:(.text+0x0): multiple definition of `syscall'
libc/libc_so.a(syscall-eabi.os):(.text+0x0): first defined here

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>