[gdb/testsuite] Fix gdb.base/infoline-reloc-main-from-zero.exp with clang

[gdb/testsuite] Fix gdb.base/infoline-reloc-main-from-zero.exp with clang

With test-case gdb.base/infoline-reloc-main-from-zero.exp and clang I run into:
...
gdb compile failed, clang-13.0: warning: -e main: 'linker' input unused \
  [-Wunused-command-line-argument]
clang-13.0: warning: -Wl,-Ttext=0x00: 'linker' input unused \
  [-Wunused-command-line-argument]
clang-13.0: warning: -Wl,-N: 'linker' input unused \
  [-Wunused-command-line-argument]
UNTESTED: gdb.base/infoline-reloc-main-from-zero.exp: \
  infoline-reloc-main-from-zero.exp
UNTESTED: gdb.base/infoline-reloc-main-from-zero.exp: failed to compile
...

Fix this by using ldflags instead of additional_flags.

Likewise, fix all occurrences of:
...
$ find gdb/testsuite -name *.exp | xargs grep additional_flags.*Wl
...

Tested on x86_64-linux.