add a compatibilty test for evaluating expressions

This is a common example, where the ADC is read multiple times (oversampled)
and the oversampling factor is calculated by a shift-left, which makes
calculating the average of samples as easy as shifting their sum right by
the same amount of bits. Using .set directives with expressions makes
the oversampling factor easily configurable.

Author: wnienhaus

tests/compat/expr.S

@@ -0,0 +1,30 @@
+  .set adc_channel, 6
+
+  .set adc_oversampling_factor_log, 2
+  .set adc_oversampling_factor, (1 << adc_oversampling_factor_log)
+
+.data
+
+result:
+  .long 0
+
+  .text
+  .global entry
+entry:
+  move r0, 0
+  stage_rst
+
+measure:
+  adc r1, 0, adc_channel + 1
+  add r0, r0, r1
+
+  stage_inc 1
+  jumps measure, adc_oversampling_factor, lt
+
+  rsh r0, r0, adc_oversampling_factor_log
+
+  move r3, result
+  st r0, r3, 0
+
+exit:
+  halt