[gdb/build] Fix Wimplicit-exception-spec-mismatch in clang build

When building with clang 13 (and -std=gnu++17 to work around an issue in
string_view-selftests.c), we run into a few Wimplicit-exception-spec-mismatch
warnings:
...
src/gdbsupport/new-op.cc:102:1: error: function previously declared with an \
  explicit exception specification redeclared with an implicit exception \
  specification [-Werror,-Wimplicit-exception-spec-mismatch]
operator delete (void *p)
^
/usr/include/c++/11/new:130:6: note: previous declaration is here
void operator delete(void*) _GLIBCXX_USE_NOEXCEPT
     ^
...

These are due to recent commit 5fff6115fea "Fix
LD_PRELOAD=/usr/lib64/libasan.so.6 gdb".

Fix this by adding the missing noexcept.

Also fix a few Wmissing-prototypes warnings by adding the missing prototype.

Build on x86_64-linux, using gcc 7.5.0 and clang 13.0.0.

---
 gdbsupport/new-op.cc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/gdbsupport/new-op.cc b/gdbsupport/new-op.cc
index 2f4c71457b1..ac7bc146bf3 100644
--- a/gdbsupport/new-op.cc
+++ b/gdbsupport/new-op.cc
@@ -99,7 +99,7 @@ operator new[] (std::size_t sz, const std::nothrow_t&) noexcept
    errors from AddressSanitizers.  */
 
 void
-operator delete (void *p)
+operator delete (void *p) noexcept
 {
   free (p);
 }
@@ -110,14 +110,19 @@ operator delete (void *p, const std::nothrow_t&) noexcept
   return ::operator delete (p);
 }
 
+extern void
+operator delete (void *p, std::size_t) noexcept;
+
 void
 operator delete (void *p, std::size_t) noexcept
 {
   return ::operator delete (p, std::nothrow);
 }
 
+extern void operator delete[] (void *p) noexcept;
+
 void
-operator delete[] (void *p)
+operator delete[] (void *p) noexcept
 {
   return ::operator delete (p);
 }
@@ -128,6 +133,8 @@ operator delete[] (void *p, const std::nothrow_t&) noexcept
   return ::operator delete (p, std::nothrow);
 }
 
+extern void operator delete[] (void *p, std::size_t) noexcept;
+
 void
 operator delete[] (void *p, std::size_t) noexcept
 {