test(port_riscv): place the SBI entry point at the base address

Fixes S-mode target tests failing on QEMU 7.0.0, where [a change][1] was
introduced to use the image base address as the entry point and ignore
the one specified by an ELF header.

[1]: https://gitlab.com/qemu-project/qemu/-/commit/7e322a7f23a60b0e181b55ef722fdf390ec4e463

Author: yvt

src/r3_port_riscv_test_driver/src/main.rs

@@ -90,6 +90,7 @@ macro_rules! instantiate_test {
         port::use_rt!(unsafe SystemTraits);
 
         #[cfg(feature = "boot-minimal-s")]
+        #[link_section = ".text_pre"]
         #[no_mangle]
         #[naked]
         extern "C" fn start() {

src/r3_test_runner/src/targets.rs

@@ -81,6 +81,7 @@ impl LinkerScripts {
 
               .text :
               {
+                KEEP(*(.text_pre .text_pre.*));
                 *(.text .text.*);
                 . = ALIGN(4);
                 __etext = .;