opcodes: LoongArch: add "ret" instruction to reduce typing

This syntactic sugar is present in both classical and emerging
architectures, like Alpha, SPARC and RISC-V, and assembler macros
doing the same thing can already be found in the wild e.g. [1], proving
the feature's popularity. It's better to provide support directly in the
assembler so downstream users wouldn't have to re-invent this over and
over again.

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/loongarch/sysdep.h;h=c586df819cd90;hb=HEAD#l28

gas/testsuite/gas/loongarch/jmp_op.d

@@ -28,3 +28,4 @@ Disassembly of section .text:
 [ 	]+48:[ 	]+6bffb8a4[ 	]+[ 	]+bltu[ 	]+\$a1, \$a0, -72\(0x3ffb8\)[ 	]+# 0x0
 [ 	]+4c:[ 	]+6fffb485[ 	]+[ 	]+bgeu[ 	]+\$a0, \$a1, -76\(0x3ffb4\)[ 	]+# 0x0
 [ 	]+50:[ 	]+6fffb0a4[ 	]+[ 	]+bgeu[ 	]+\$a1, \$a0, -80\(0x3ffb0\)[ 	]+# 0x0
+[ 	]+54:[ 	]+4c000020[ 	]+[ 	]+jirl[ 	]+\$zero, \$ra, 0

gas/testsuite/gas/loongarch/jmp_op.s

@@ -20,3 +20,4 @@ bltu  $r4,$r5,.L1
 bgtu  $r4,$r5,.L1
 bgeu  $r4,$r5,.L1
 bleu  $r4,$r5,.L1
+ret

ld/testsuite/ld-loongarch-elf/jmp_op.d

@@ -28,3 +28,4 @@ Disassembly of section .text:
 [ 	]+48:[ 	]+6bffb8a4[ 	]+[ 	]+bltu[ 	]+\$a1, \$a0, -72\(0x3ffb8\)[ 	]+# 0x0
 [ 	]+4c:[ 	]+6fffb485[ 	]+[ 	]+bgeu[ 	]+\$a0, \$a1, -76\(0x3ffb4\)[ 	]+# 0x0
 [ 	]+50:[ 	]+6fffb0a4[ 	]+[ 	]+bgeu[ 	]+\$a1, \$a0, -80\(0x3ffb0\)[ 	]+# 0x0
+[ 	]+54:[ 	]+4c000020[ 	]+[ 	]+jirl[ 	]+\$zero, \$ra, 0

ld/testsuite/ld-loongarch-elf/jmp_op.s

@@ -20,3 +20,4 @@ bltu  $r4,$r5,.L1
 bgtu  $r4,$r5,.L1
 bgeu  $r4,$r5,.L1
 bleu  $r4,$r5,.L1
+ret

opcodes/loongarch-opc.c

@@ -841,6 +841,7 @@ static struct loongarch_opcode loongarch_jmp_opcodes[] =
   { 0x0,	0x0,		"bgtu",		"r,r,la",			"bltu %2,%1,%%b16(%3)",		0, 0, 0 },
   { 0x0,	0x0,		"bleu",		"r,r,la",			"bgeu %2,%1,%%b16(%3)",		0, 0, 0 },
   { 0x0,	0x0,		"jr",		"r",				"jirl $r0,%1,0",		0, 0, 0 },
+  { 0x0,	0x0,		"ret",		"",				"jirl $r0,$r1,0",		0, 0, 0 },
   { 0 } /* Terminate the list.  */
 };