[GDBserver] Use regcache_read_pc in install_software_single_step_breakpoints

[GDBserver] Use regcache_read_pc in install_software_single_step_breakpoints

In install_software_single_step_breakpoints, we've got the regcache
of current_thread, so we don't have to bother get_pc to get pc,
instead we can get pc from regcache directly.  Note that the callers
of install_software_single_step_breakpoints have already switched
current_thread to LWP.

Since the pc is got from regcache_read_pc, in the next patch, we can
get pc inside the implementation of *the_low_target.get_next_pcs and
stop passing pc to *the_low_target.get_next_pcs.

gdb/gdbserver:

2016-01-26  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (install_software_single_step_breakpoints): Call
	regcache_read_pc instead of get_pc.