revise LDELFFLAGS

[yf13]

Summary

When building maix-bit/elf, we got warnings like below, depicting that __start is still used for some ELF cases:

LD: errno.o
riscv64-unknown-elf-ld: warning: cannot find entry symbol __start; defaulting to 0000000000000000

This aims to resolve such warnings and use _start for executable ELFs.

New config rv-virt:elf similar to maix-bit/elf is added for verifying the change without using actual hardware.

Impact

Bug fixing that should not bring behavior differences.

Testing

• Local checks on rv-virt for the following configs:
  • Build and run checks: rv-virt:nsh, rv-virt:pnsh, rv-virt:nsbi and ri-virt:elf
  • Build only check: maix-bit/elf
• CI checks