Make Position Independent Executables work for CloudABI.

- Set BI_CAN_EXEC_DYN, so we can execute ET_DYN ELF files in addition to
  regular ET_EXECs.
- Provide an AT_BASE entry in the auxiliary vector, so the executable
  knows at which address it got loaded and can apply relocations.

sys/amd64/cloudabi64/cloudabi64_sysvec.c

@@ -156,5 +156,6 @@ Elf64_Brandinfo cloudabi64_brand = {
 	.brand		= ELFOSABI_CLOUDABI,
 	.machine	= EM_X86_64,
 	.sysvec		= &cloudabi64_elf_sysvec,
+	.flags		= BI_CAN_EXEC_DYN,
 	.compat_3_brand	= "CloudABI",
 };

sys/arm64/cloudabi64/cloudabi64_sysvec.c

@@ -157,5 +157,6 @@ Elf64_Brandinfo cloudabi64_brand = {
 	.brand		= ELFOSABI_CLOUDABI,
 	.machine	= EM_AARCH64,
 	.sysvec		= &cloudabi64_elf_sysvec,
+	.flags		= BI_CAN_EXEC_DYN,
 	.compat_3_brand	= "CloudABI",
 };

sys/compat/cloudabi64/cloudabi64_module.c

@@ -99,6 +99,7 @@ cloudabi64_fixup(register_t **stack_base, struct image_params *imgp)
 #define	PTR(type, ptr)	{ .a_type = (type), .a_ptr = (uintptr_t)(ptr) }
 		PTR(CLOUDABI_AT_ARGDATA, argdata),
 		VAL(CLOUDABI_AT_ARGDATALEN, argdatalen),
+		VAL(CLOUDABI_AT_BASE, args->base),
 		PTR(CLOUDABI_AT_CANARY, canary),
 		VAL(CLOUDABI_AT_CANARYLEN, sizeof(canarybuf)),
 		VAL(CLOUDABI_AT_NCPUS, mp_ncpus),