[gdb/tdep, rs6000] Don't skip system call in skip_prologue

[gdb/tdep, rs6000] Don't skip system call in skip_prologue

I ran into a case where a breakpoint on _exit never triggered, because it was
set past the end of the _exit prologue, past the end of the exit_group system
call (which does not return).

More concretely, the breakpoint was set at the last insn show here:
...
Dump of assembler code for function _exit:
   0x00007ffff7e42ea0 <+0>:     12 00 4c 3c     addis   r2,r12,18
   0x00007ffff7e42ea4 <+4>:     60 43 42 38     addi    r2,r2,17248
   0x00007ffff7e42ea8 <+8>:     00 00 00 60     nop
   0x00007ffff7e42eac <+12>:    f8 ff e1 fb     std     r31,-8(r1)
   0x00007ffff7e42eb0 <+16>:    78 1b 7f 7c     mr      r31,r3
   0x00007ffff7e42eb4 <+20>:    f0 ff c1 fb     std     r30,-16(r1)
   0x00007ffff7e42eb8 <+24>:    ea 00 00 38     li      r0,234
   0x00007ffff7e42ebc <+28>:    a0 8b 22 e9     ld      r9,-29792(r2)
   0x00007ffff7e42ec0 <+32>:    78 fb e3 7f     mr      r3,r31
   0x00007ffff7e42ec4 <+36>:    14 6a c9 7f     add     r30,r9,r13
   0x00007ffff7e42ec8 <+40>:    02 00 00 44     sc
   0x00007ffff7e42ecc <+44>:    26 00 00 7c     mfcr    r0
   0x00007ffff7e42ed0 <+48>:    00 10 09 74     andis.  r9,r0,4096
...

Fix this by treating system calls the same as branches in skip_prologue:
by default, don't skip, such that the breakpoint is set at 0x00007ffff7e42eb8
instead.

Tested on ppc64le-linux, on a power 8 machine.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28527