py/emitinlinerv32: Add inline assembler support for RV32.

This commit adds support for writing inline assembler functions when
targeting a RV32IMC processor.

Given that this takes up a bit of rodata space due to its large
instruction decoding table and its extensive error messages, it is
enabled by default only on offline targets such as mpy-cross and the
qemu port.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tests/inlineasm/rv32/asmdata.py

@@ -0,0 +1,33 @@
+# test the "data" directive
+
+
+@micropython.asm_rv32
+def ret_num(a0) -> uint:
+    slli(a0, a0, 2)
+    addi(a0, a0, 16)
+    auipc(a1, 0)
+    add(a1, a1, a0)
+    lw(a0, 0(a1))
+    jal(zero, HERE)
+    data(4, 0x12345678, 0x20000000, 0x40000000, 0x7FFFFFFF + 1, (1 << 32) - 2)
+    label(HERE)
+
+
+for i in range(5):
+    print(hex(ret_num(i)))
+
+
+@micropython.asm_rv32
+def ret_num_la(a0) -> uint:
+    slli(a0, a0, 2)
+    la(a1, DATA)
+    add(a1, a1, a0)
+    lw(a0, 0(a1))
+    jal(zero, HERE)
+    label(DATA)
+    data(4, 0x12345678, 0x20000000, 0x40000000, 0x7FFFFFFF + 1, (1 << 32) - 2)
+    label(HERE)
+
+
+for i in range(5):
+    print(hex(ret_num_la(i)))