Fix build failure for 32-bit targets

Fix build failure for 32-bit targets

When building master GDB, I ran into the following:

binutils-gdb/gdb/bt-utils.c: In function 'int libbacktrace_print(void*, uintptr_t, const char*, int, const char*)':
binutils-gdb/gdb/bt-utils.c:93:44: error: format '%lx' expects argument of type 'long unsigned int', but argument 4 has type 'uintptr_t {aka unsigned int}' [-Werror=format=]
   snprintf (buf, sizeof (buf), "0x%lx ", pc);

Fix this by using %PRIxPTR as opposed to %lx.