[MIPS] Drop .dynsym symbol ordering requirement and nullify DT_GNU_XHASH

[Fangrui Song]

In August 2019, GNU ld gained support for DT_GNU_XHASH [1] (DT_GNU_HASH with an additional symbol index translation table (xlat)).
A gold patch [2] was posted but has not been applied.

https://sourceware.org/git/?p=glibc.git;a=commit;h=23c1c256ae7b0f010d0fcaff60682b620887b164 added glibc ld.so support.

Instead of diverging more from other architectures, I wonder whether we can drop .dynsym symbol ordering requirement, and just use regular DT_GNU_HASH. To this end, DT_MIPS_GOTSYM=DT_MIPS_SYMTABNO and DT_MIPS_LOCAL_GOTNO=0 should be sufficient for musl [3].

As I understand it, the benefits of the symbol ordering requirement come from the following:

1.  DT_MIPS_LOCAL_GOTNO relative relocation entries are saved. Relative relocations are common for -pie, but I really doubt the efficiency of DT_MIPS_LOCAL_GOTNO. Many relative relocations are not associated with a dynamic symbol. Adopting the unofficial SHT_RELR/DT_RELR [4] may be more beneficial.
2.  DT_MIPS_SYMTABNO-DT_MIPS_GOTSYM words are saved for JUMP_SLOT. The saving is also questionable because the number of JUMP_SLOT is usually small.
3. The DT_GNU_XHASH translation table seems to partly defeat the size saving purposes of 2. :)

Other than DT_MIPS_GOTSYM=DT_MIPS_SYMTABNO and DT_MIPS_LOCAL_GOTNO=0, I don't know whether glibc ld.so needs more things so that old ld.so can refuse new executables/shared objects. There may be multiple ways. I confess I don't understand the matters and MIPS enough, but something that jumped into my mind: bumping EI_ABIVERSION, introducing a new dynamic tag DT_MIPS_DYNSYM (think ppc32 Secure-PLT DT_PPC_GOT), .MIPS.abiflags, etc.

(
DT_GNU_XHASH may complicate --hash-style=gnu --hash-style=both semantics in clang driver/gcc specs as well.)

PS: I am not a MIPS user. I just don't want to see more ELF hacks invented to further complicate toolchains...
)

[1]: binutils support https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=f16a9783c5f085443d806646074e9c06fdee9a88 
      https://sourceware.org/ml/binutils/2019-08/msg00092.html
[2]: gold patch https://sourceware.org/ml/binutils/2019-07/msg00227.html
[3]: musl ld.so https://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c?id=d01fdc777dd3b5ebcad351ee47d1984d28db31e4#n1297
[4]: SHT_RELR/DT_RELR https://groups.google.com/forum/#!search/SHT_RELR/generic-abi/bX460iggiKg/w95-EVqFBAAJ supported by llvm-readelf / lld (--pack-dyn-relocs=relr; https://reviews.llvm.org/D48247)