LoongArch: Allow la.got -> la.pcrel relaxation for shared object

LoongArch: Allow la.got -> la.pcrel relaxation for shared object

Even in shared objects, la.got -> la.pcrel relaxation can still be
performed for symbols with hidden visibility. For example, if a.c is:

    extern int x;
    int f() { return x++; }

and b.c is:

    int x = 114514;

If compiling and linking with:

    gcc -shared -fPIC -O2 -fvisibility=hidden a.c b.c

Then the la.got in a.o should be relaxed to la.pcrel, and the resulted f
should be like:

    pcaddi  $t0, x
    ldptr.w $a0, $t0, 0
    addi.w  $t1, $a0, 1
    stptr.w $t1, $t0, 0
    ret

Remove bfd_link_executable from the condition of la.got -> la.pcrel
relaxation so this will really happen.  The SYMBOL_REFERENCES_LOCAL
check is enough not to wrongly relax preemptable symbols (for e.g.
when -fvisibility=hidden is not used).

Note that on x86_64 this is also relaxed and the produced code is like:

    lea x(%rip), %rdx
    mov (%rdx), %rax
    lea 1(%rax), %ecx
    mov %ecx, (%rdx)
    ret

Tested by running ld test suite, bootstrapping and regtesting GCC with
the patched ld, and building and testing Glibc with the patched ld.  No
regression is observed.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>