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

[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.

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