pmc: Use isascii(3) to check if a character is ASCII

The previous check did not work on architectures where `char` is
unsigned as noted by GCC on aarch64:

usr.sbin/pmc/view.cc: In member function 'void pmcview::loadsymboltable(image*, Elf*, Elf_Scn*, GElf_Shdr*)':
usr.sbin/pmc/view.cc:627:38: error: comparison is always false due to limited range of data type [-Werror=type-limits]
  627 |                         if (fname[i] < 0) {
      |                             ~~~~~~~~~^~~

Reported by:	GCC 15

usr.sbin/pmc/view.cc

@@ -624,7 +624,7 @@ pmcview::loadsymboltable(image *im, Elf *e, Elf_Scn *scn, GElf_Shdr *sh)
 
 		// XXX: Extra checks to make sure we don't get corrupted
 		for (int i = 0; fname[i] != 0 && i < 32; i++) {
-			if (fname[i] < 0) {
+			if (!isascii(fname[i])) {
 				printf("EEEK SYMBOL\n");
 				printf("%s\n", fname);
 				assert(false);