x86/crypto/sha1_avx2: Standardize stack alignment prologue

Use a more standard prologue for saving the stack pointer before
realigning the stack.

This enables ORC unwinding by allowing objtool to understand the stack
realignment.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/fdaaf8670ed1f52f55ba9a6bbac98c1afddc1af6.1614182415.git.jpoimboe@redhat.com

arch/x86/crypto/sha1_avx2_x86_64_asm.S

@@ -645,9 +645,9 @@ _loop3:
 	RESERVE_STACK  = (W_SIZE*4 + 8+24)
 
 	/* Align stack */
-	mov	%rsp, %rbx
+	push	%rbp
+	mov	%rsp, %rbp
 	and	$~(0x20-1), %rsp
-	push	%rbx
 	sub	$RESERVE_STACK, %rsp
 
 	avx2_zeroupper
@@ -665,8 +665,8 @@ _loop3:
 
 	avx2_zeroupper
 
-	add	$RESERVE_STACK, %rsp
-	pop	%rsp
+	mov	%rbp, %rsp
+	pop	%rbp
 
 	pop	%r15
 	pop	%r14