[gdb/testsuite] Fix base address selection entry encoding in dw2-skip-prologue.S

[gdb/testsuite] Fix base address selection entry encoding in dw2-skip-prologue.S

A base address selection entry in a location list consist of two (constant or
relocated) address offsets.  The two offsets are the same size as an address
on the target machine.

The test-case gdb.dwarf2/dw2-skip-prologue.S encodes a base address selection
entry using .4byte, which is incorrect for 8-byte pointer size. [ Which
triggers an assert in dwz, see PR dwz/24172. ]

Fix this by using PTRBYTE instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-05-14  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-skip-prologue.S (.debug_loc): Fix base address
	selection entry encoding.