Convert char array to std::string in linux_find_memory_regions_full

Convert char array to std::string in linux_find_memory_regions_full

This is a quick cleanup that removes the use of fixed-length char arrays and
uses std::string instead.

gdb/ChangeLog:

2021-03-24  Luis Machado  <luis.machado@linaro.org>

	* linux-tdep.c (linux_find_memory_regions_full): Use std::string
	instead of char arrays.