riscv32: add thread support

Identical to riscv64 except for stack offsets in clone.

src/thread/riscv32/__set_thread_area.s

@@ -0,0 +1,6 @@
+.global __set_thread_area
+.type   __set_thread_area, %function
+__set_thread_area:
+	mv tp, a0
+	li a0, 0
+	ret

src/thread/riscv32/__unmapself.s

@@ -0,0 +1,7 @@
+.global __unmapself
+.type __unmapself, %function
+__unmapself:
+	li a7, 215 # SYS_munmap
+	ecall
+	li a7, 93  # SYS_exit
+	ecall

src/thread/riscv32/clone.s

@@ -0,0 +1,34 @@
+# __clone(func, stack, flags, arg, ptid, tls, ctid)
+#           a0,    a1,    a2,  a3,   a4,  a5,   a6
+
+# syscall(SYS_clone, flags, stack, ptid, tls, ctid)
+#                a7     a0,    a1,   a2,  a3,   a4
+
+.global __clone
+.type  __clone, %function
+__clone:
+	# Save func and arg to stack
+	addi a1, a1, -16
+	sw a0, 0(a1)
+	sw a3, 4(a1)
+
+	# Call SYS_clone
+	mv a0, a2
+	mv a2, a4
+	mv a3, a5
+	mv a4, a6
+	li a7, 220 # SYS_clone
+	ecall
+
+	beqz a0, 1f
+	# Parent
+	ret
+
+	# Child
+1:      lw a1, 0(sp)
+	lw a0, 4(sp)
+	jalr a1
+
+	# Exit
+	li a7, 93 # SYS_exit
+	ecall

src/thread/riscv32/syscall_cp.s

@@ -0,0 +1,29 @@
+.global __cp_begin
+.hidden __cp_begin
+.global __cp_end
+.hidden __cp_end
+.global __cp_cancel
+.hidden __cp_cancel
+.hidden __cancel
+.global __syscall_cp_asm
+.hidden __syscall_cp_asm
+.type __syscall_cp_asm, %function
+__syscall_cp_asm:
+__cp_begin:
+	lw t0, 0(a0)
+	bnez t0, __cp_cancel
+
+	mv t0, a1
+	mv a0, a2
+	mv a1, a3
+	mv a2, a4
+	mv a3, a5
+	mv a4, a6
+	mv a5, a7
+	lw a6, 0(sp)
+	mv a7, t0
+	ecall
+__cp_end:
+	ret
+__cp_cancel:
+	tail __cancel