gcc: Make strchr return value pointers const

This fixes compilation of codepaths for dos-like filesystems
with Clang. When built with clang, it treats C input files as C++
when the compiler driver is invoked in C++ mode, triggering errors
when the return value of strchr() on a pointer to const is assigned
to a pointer to non-const variable.

This matches similar variables outside of the ifdefs for dos-like
path handling.

2020-09-07  Martin Storsjö  <martin@martin.st>

gcc/
	* dwarf2out.c (file_name_acquire): Make a strchr return value
	pointer to const.
libcpp/
	* files.c (remap_filename): Make a strchr return value pointer
	to const.