arch/xtensa/src/esp32/esp32_user.c: Implement L16SI emulation
I don't know why this was not necessary before.
Probably I was just lucky about the combination of configs.
Or maybe some of recent changes happened to make the compiler
to use the instruction.
```
400d38f0 <mm_givesemaphore>:
400d38f0: 004136 entry a1, 32
400d38f3: 228c beqz.n a2, 400d38f9 <mm_givesemaphore+0x9>
400d38f5: 0228 l32i.n a2, a2, 0
400d38f7: 52cc bnez.n a2, 400d3900 <mm_givesemaphore+0x10>
400d38f9: fea0b2 movi a11, 254
400d38fc: 000306 j 400d390c <mm_givesemaphore+0x1c>
400d38ff: 00 .byte 00
400d3900: 019232 l16si a3, a2, 2
400d3903: feebe5 call8 400d27c0 <getpid>
400d3906: 0813a7 beq a3, a10, 400d3912 <mm_givesemaphore+0x22>
400d3909: 05a1b2 movi a11, 0x105
400d390c: f241a1 l32r a10, 400d0210 <_stext+0x1f0>
400d390f: ff23e5 call8 400d2b4c <_assert>
400d3912: 1288 l32i.n a8, a2, 4
400d3914: 0828a6 blti a8, 2, 400d3920 <mm_givesemaphore+0x30>
400d3917: 880b addi.n a8, a8, -1
400d3919: 1289 s32i.n a8, a2, 4
400d391b: 000606 j 400d3937 <mm_givesemaphore+0x47>
400d391e: 00 .byte 00
400d391f: 00 .byte 00
400d3920: ffaf82 movi a8, -1
400d3923: 015282 s16i a8, a2, 2
400d3926: 00a082 movi a8, 0
400d3929: 016282 s32i a8, a2, 4
400d392c: 02ad mov.n a10, a2
400d392e: feb125 call8 400d2440 <sem_post>
400d3931: 19a1b2 movi a11, 0x119
400d3934: fd4a96 bltz a10, 400d390c <mm_givesemaphore+0x1c>
400d3937: f01d retw.n
400d3939: 000000 ill
```
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