firmware: do not use PIC in Rust code.

This fixes test_pulse_rate and test_pulse_rate_dds by making
rtio_output slightly faster.

artiq/runtime/Makefile

@@ -28,12 +28,18 @@ LDFLAGS += --gc-sections \
 	-L../libunwind \
 	-L../libdyld
 
+RUSTFLAGS = \
+	-C target-feature=+mul,+div,+ffl1,+cmov,+addc \
+	-C relocation-model=static \
+	-C opt-level=s \
+	-Cpanic=abort
+export RUSTFLAGS
+
 all: runtime.bin runtime.fbi
 
 .PHONY: $(RUSTOUT)/libruntime.a
 $(RUSTOUT)/libruntime.a:
 	$(CARGO) \
-		RUSTFLAGS="-C target-feature=+mul,+div,+ffl1,+cmov,+addc -C opt-level=s -Cpanic=abort" \
 		CARGO_TARGET_DIR=$(realpath .)/cargo \
 			cargo build --target=or1k-unknown-none \
 				--manifest-path $(realpath $(RUNTIME_DIRECTORY)/../firmware/runtime/Cargo.toml)
@@ -49,7 +55,6 @@ runtime.elf: $(RUSTOUT)/libruntime.a ksupport_data.o
 .PHONY: $(RUSTOUT_KSUPPORT)/libksupport.a
 $(RUSTOUT_KSUPPORT)/libksupport.a:
 	$(CARGO) \
-		RUSTFLAGS="-C target-feature=+mul,+div,+ffl1,+cmov,+addc -C opt-level=s -Cpanic=unwind" \
 		CARGO_TARGET_DIR=$(realpath .)/cargo-ksupport \
 			cargo build --target=or1k-unknown-none \
 				--manifest-path $(realpath $(RUNTIME_DIRECTORY)/../firmware/ksupport/Cargo.toml)

artiq/test/coredevice/test_rtio.py

@@ -356,7 +356,6 @@ class CoredeviceTest(ExperimentCase):
         self.assertGreater(rate, 100*ns)
         self.assertLess(rate, 700*ns)
 
-    @unittest.skip("fails on CI for unknown reasons")
     def test_pulse_rate_dds(self):
         """Minimum interval for sustained DDS frequency switching"""
         self.execute(PulseRateDDS)