[gdb/testsuite] Fix gdb.base/watchpoint-reuse-slot.exp with gcc-8

[gdb/testsuite] Fix gdb.base/watchpoint-reuse-slot.exp with gcc-8

When running test-case gdb.base/watchpoint-reuse-slot.exp with gcc-8 instead
of gcc-7, we have:
...
 (gdb) PASS: $conf: watch *(buf.byte + 0 + 0)@1
 stepi^M
-0x00000000004004b9      34        for (i = 0; i < 100000; i++);^M
+34        for (i = 0; i < 100000; i++);^M
-(gdb) PASS: $conf: stepi advanced
+(gdb) FAIL: $conf: stepi advanced
...
where $conf is "gdb.base/watchpoint-reuse-slot.exp: hw-watch: always-inserted
off: watch x watch: : width 1, iter 0: base + 0".

This is due to the fact that gcc-8 generates more precise line info, making
the instruction at 0x4004b9 a "recommended breakpoint location", such that gdb
no longer prints the instruction address.

Fix this by getting the instruction address by printing $pc.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-06  Tom de Vries  <tdevries@suse.de>

	* gdb.base/watchpoint-reuse-slot.exp (stepi): Print $pc to get current
	address.