gdb.base/catch-syscall.exp: Remove some Linux-only assumptions.

gdb.base/catch-syscall.exp: Remove some Linux-only assumptions.

- Some OS's use a different syscall for exit().  For example, the
  BSD's use SYS_exit rather than SYS_exit_group.  Update the C source
  file and the expect script to support SYS_exit as an alternative to
  SYS_exit_group.

- The cross-arch syscall number tests are all Linux-specific with
  hardcoded syscall numbers specific to Linux kernels.  Skip these
  tests on non-Linux systems.  FreeBSD kernels for example use the
  same system call numbers on all platforms, so the test is also not
  relevant on FreeBSD.