oss-fuzz: move linker arg to fix coverage-build

The order of the add_project_link_arguments calls impacts which
arguments are placed between --start-group and --end-group.
OSS-Fuzz coverage builds seem to just add these to CFLAGS:
-fprofile-instr-generate -fcoverage-mapping pthread -Wl,--no-as-needed
-Wl,-ldl -Wl,-lm Wno-unused-command-line-argument

The -Wl,-ldl flag that is enough to shift the fork_fuzz.ld linker-script
back into the linker group. Move the linker-script meson call before the
other calls to make sure the flag is placed correctly.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20200909220516.614222-1-alxndr@bu.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>