gdb, testsuite: Fix argument unused warning with clang

Since clang 19 we see:
~~
clang: warning: argument unused during compilation: '-nostartfiles' [-Wunused-command-line-argument]^M
gdb compile failed, clang: warning: argument unused during compilation: '-nostartfiles' [-Wunused-command-line-argument]
UNTESTED: gdb.arch/amd64-init-x87-values.exp: failed to prepare
~~

Fix this by only passing '-nostartfiles' when linking.

Approved-By: Tom Tromey <tom@tromey.com>