SHA512/x86_64: Fix SIGSEGV on $avx=0 path

SHA512/x86_64: Fix SIGSEGV on $avx=0 path

Fixes #27555

When $avx>1, codegen is:
    lea OPENSSL_ia32cap_P(%rip),%r10
    mov 0(%r10),%r9
    mov 8(%r10),%r11d
    mov 20(%r10),%r10d

When $avx is true, codegen was:
    lea OPENSSL_ia32cap_P(%rip),%r10
    mov 0(%r10),%r9
    mov 8(%r10),%r11d
    mov 20(%r10),%r10d    # not used by dispatcher

When $avx=0, codegen was:
    mov 20(%r10),%r10d    # %r10 not initalized, causing SIGSEGV

After fix, `mov 20(%r10),%r10d` is only included when $avx>1

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27574)