gdb: pass std::string from linux_find_memory_regions_full

Update linux_find_memory_region_ftype to take 'const std::string &'
instead of 'const char *', update the two functions which are passed
as callbacks to linux_find_memory_regions_full.

There should be no user visible changes after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>