LoongArch: Fix the condition to use PC-relative addressing in start.S

LoongArch: Fix the condition to use PC-relative addressing in start.S

A start.o compiled from start.S with -DPIC and no -DSHARED is used by
both crt1.o and rcrt1.o.  So the LoongArch static PIE patch
unintentionally introduced PC-relative addressing for main and
__libc_start_main into crt1.o.

While the latest Binutils (trunk, which will be released as 2.40)
supports the PC-relative relocs against an external function by creating
a PLT entry, the 2.39 release branch doesn't (and won't) support this.
An error is raised:

    "PLT stub does not represent and symbol not defined."

So, we need the following changes:

1. Check if ld supports the PC-relative relocs against an external
   function.  If it's not supported, we deem static PIE unsupported.
2. Change start.S.  If static PIE is supported, use PC-relative
   addressing for main and __libc_start_main and rely on the linker to
   create PLT entries.  Otherwise, restore the old behavior (using GOT
   to address these functions).

An alternative would be adding a new "static-pie-start.S", and some
custom logic into Makefile to build rcrt1.o with it.  And, restore
start.S to the state before static PIE change so crt1.o won't contain
PC-relative relocs against external symbols.  But I can't see any
benefit of this alternative, so I'd just keep it simple.

Tested by building glibc with the following configurations:

1. Binutils trunk + GCC trunk.  Static PIE enabled.  All tests
   passed.
2. Binutils 2.39 branch + GCC trunk.  Static PIE disabled.  Tests
   related to ifunc failed (it's a known issue).  All other tests
   passed.
3. Binutils 2.39 branch + GCC 12 branch, cross compilation with
   build-many-glibcs.py from x86_64-linux-gnu.  Static PIE disabled.
   Build succeeded.