[platform][sifive-e] get the hifive1 hardware working again

Had rotted a bit. Had to reimplement a few tweaks.
Also generally #if out a lot of the SMP code when unused on riscv.

target/sifive-e/rules.mk

@@ -4,6 +4,7 @@ MODULE := $(LOCAL_DIR)
 
 PLATFORM := sifive
 VARIANT := sifive_e
+WITH_LINKER_GC ?= 1
 
 MEMSIZE ?= 0x4000     # 16KB
 GLOBAL_DEFINES += TARGET_HAS_DEBUG_LED=1

target/sifive-e/target.c

@@ -26,6 +26,8 @@ void target_early_init(void) {
     // program the pll bypass, we should be running at 16Mhz now
     prci_base[2] = 0x00070df1;
 
+    // lfclock is a 32768Hz crystal, strapped externally
+
     // io function enable for pin 16/17, no IOF for all others
     gpio_base[14] = (3<<16);