x86-64: Fix the tcb field load for x32 [BZ #31185]

_dl_tlsdesc_undefweak and _dl_tlsdesc_dynamic access the thread pointer
via the tcb field in TCB:

_dl_tlsdesc_undefweak:
        _CET_ENDBR
        movq    8(%rax), %rax
        subq    %fs:0, %rax
        ret

_dl_tlsdesc_dynamic:
	...
        subq    %fs:0, %rax
        movq    -8(%rsp), %rdi
        ret

Since the tcb field in TCB is a pointer, %fs:0 is a 32-bit location,
not 64-bit. It should use "sub %fs:0, %RAX_LP" instead.  Since
_dl_tlsdesc_undefweak returns ptrdiff_t and _dl_make_tlsdesc_dynamic
returns void *, RAX_LP is appropriate here for x32 and x86-64.  This
fixes BZ #31185.

(cherry picked from commit 81be2a61)