build: improve linker script and target settings

[josephlr]

This change is a few small fixes to target.json and layout.ld

• Set stripping in layout.ld
  • Fixes my mistake from Update toolchain, improve linker script, and reduce binary size. #20
  • We already set linking flags in layout.ld, we shouldn't also be setting them in a second place.
  • I updated the comments to explain why we can also do this for panic = "abort"
    • Without setting the default target to be target.json, cargo check and cargo clippy will break when building for the host target.
• Merge .bss into .data
  • This .bss section normally represents "uninitialized/zero" data that will later be written by the program. Generally this means it has a runtime memory area, but no corresponding data in the output binary. The program header just says where to load it.
  • To make the loader's job easier (including the custom loader we will need for Support building as flat BIOS image #5), we put the .bss section in .data. This now puts explicitly zeroed data in .data instead of relying on the loader to "expand" the .bss data at runtime.
  • EDK2 uses a similar trick
  • This doesn't currently have an effect on binary size, as ld.ldd (unlike the gnutools ld) just directly zeros the .bss region in the file. However, this isn't a stable behavior to rely on.
• Set more parameters in target.json
  • As we're building a firmware, we have no need for Rust's (on by default) Position Independent Code (PIC). This is controlled by the relocation-mode option.
  • We also explicitly set code-model and dynamic-linking-available to make our linking assumptions clear.
  • This makes the binary a little smaller and a little faster, but the main reason to do this is to making linking simpler.

Signed-off-by: Joe Richey [email protected]