Fix failed tests related to pointer printing when using GDB 10

As mentioned in #79009, there are four failed debuginfo test cases
when using GDB 10. This commit fixes two of them, which fail because
GDB 10 won't print pointers as string anymore. We can use `printf`
as a workaround. It should work regardless of the version of GDB.

Refer this [comment] for more details.

[comment]: https://github.com/rust-lang/rust/issues/79009#issuecomment-826952708

src/test/debuginfo/extern-c-fn.rs

@@ -5,17 +5,16 @@
 // === GDB TESTS ===================================================================================
 // gdb-command:run
 
-// gdb-command:print s
-// gdbg-check:$1 = [...]"abcd"
-// gdbr-check:$1 = [...]"abcd\000"
+// gdb-command:printf "s = \"%s\"\n", s
+// gdb-check:s = "abcd"
 // gdb-command:print len
-// gdb-check:$2 = 20
+// gdb-check:$1 = 20
 // gdb-command:print local0
-// gdb-check:$3 = 19
+// gdb-check:$2 = 19
 // gdb-command:print local1
-// gdb-check:$4 = true
+// gdb-check:$3 = true
 // gdb-command:print local2
-// gdb-check:$5 = 20.5
+// gdb-check:$4 = 20.5
 
 // gdb-command:continue
 

src/test/debuginfo/pretty-huge-vec.rs

@@ -13,7 +13,7 @@
 // gdb-check:$1 = Vec(size=1000000000) = {[...]...}
 
 // gdb-command: print slice
-// gdb-check:$2 = &[u8] {data_ptr: [...]"\000", length: 1000000000}
+// gdb-check:$2 = &[u8] {data_ptr: [...], length: 1000000000}
 
 #![allow(unused_variables)]