gdb: Support stepping out from signal handler on riscv*-linux

Currently, gdb cannot step outside of a signal handler on RISC-V
platforms.  This causes multiple failures in gdb.base/sigstep.exp:

	FAIL: gdb.base/sigstep.exp: continue to handler, nothing in handler, step from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, si+advance in handler, step from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, nothing in handler, next from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, si+advance in handler, next from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: stepi from handleri: leave signal trampoline
	FAIL: gdb.base/sigstep.exp: nexti from handleri: leave signal trampoline

	                === gdb Summary ===

	# of expected passes            587
	# of unexpected failures        6

This patch adds support for stepping outside of a signal handler on
riscv*-*-linux*.

Implementation is heavily inspired from mips_linux_syscall_next_pc and
surroundings as advised by Pedro Alves.

After this patch, all tests in gdb.base/sigstep.exp pass.

Build and tested on riscv64-linux-gnu.

1 files changed, 4 insertions, 0 deletions

diff --git a/gdb/riscv-tdep.h b/gdb/riscv-tdep.h
index 62bf479..03870cf 100644
--- a/gdb/riscv-tdep.h
+++ b/gdb/riscv-tdep.h
@@ -34,6 +34,7 @@ enum
   RISCV_FP_REGNUM = 8,		/* Frame Pointer.  */
   RISCV_A0_REGNUM = 10,		/* First argument.  */
   RISCV_A1_REGNUM = 11,		/* Second argument.  */
+  RISCV_A7_REGNUM = 17,		/* Seventh argument.  */
   RISCV_PC_REGNUM = 32,		/* Program Counter.  */
 
   RISCV_NUM_INTEGER_REGS = 32,
@@ -102,6 +103,9 @@ struct gdbarch_tdep
   int duplicate_frm_regnum = -1;
   int duplicate_fcsr_regnum = -1;
 
+  /* Return the expected next PC assuming FRAME is stopped at a syscall
+     instruction.  */
+  CORE_ADDR (*syscall_next_pc) (struct frame_info *frame);
 };