gdb: avoid unnecessary string copy in auto_load_objfile_script_1

Assigning the result of STRIP_DRIVE_SPEC to an std::string creates an
unnecessary copy of the string.  STRIP_DRIVE_SPEC is defined as:

  #define STRIP_DRIVE_SPEC(f) ((f) + 2)

So if it is passed a "const char *", it returns a "const char *".  We
could use a "const char *" intermediary variable instead of an
std::string, or (as implemented in this patch) just use it directly in
the concatenation right after.

gdb/ChangeLog:

	* auto-load.c (auto_load_objfile_script_1): Don't use
	debugfile_holder as temporary variable when stripping drive
	letter.

Change-Id: If2ccc7a156b22100754d9cdf6778ac7eeb93da4c