poly1305/asm/poly1305-armv4.pl: remove unintentional relocation.

Branch to global symbol results in reference to PLT, and when compiling
for THUMB-2 - in a R_ARM_THM_JUMP19 relocation. Some linkers don't
support this relocation (ld.gold), while others can end up truncating
the relocation to fit (ld.bfd).

Convert this branch through PLT into a direct branch that the assembler
can resolve locally.

See https://github.com/android-ndk/ndk/issues/337 for background.

The current workaround is to disable poly1305 optimization assembly,
which is not optimal and can be reverted after this patch:
https://github.com/freedesktop/gstreamer-cerbero/commit/beab607d2b1ff23c41b7e01aa9c64be5e247d1e6

CLA: trivial
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5949)

crypto/poly1305/asm/poly1305-armv4.pl

@@ -186,6 +186,7 @@ $code.=<<___;
 .type	poly1305_blocks,%function
 .align	5
 poly1305_blocks:
+.Lpoly1305_blocks:
 	stmdb	sp!,{r3-r11,lr}
 
 	ands	$len,$len,#-16
@@ -677,7 +678,7 @@ poly1305_blocks_neon:
 	cmp	$len,#64
 	bhs	.Lenter_neon
 	tst	ip,ip			@ is_base2_26?
-	beq	poly1305_blocks
+	beq	.Lpoly1305_blocks
 
 .Lenter_neon:
 	stmdb	sp!,{r4-r7}