Make build_symbol_table.py output statistics on the symbol table it
builds, and emit warnings for zero-length symbols. Also added lengths to
several functions defined in asm that this uncovered.
Using `-fvisibility=hidden` when building the kernel, and then
`--discard-all` when stripping it, we shave almost 100KiB off of the
resulting ELF file.
Also dropped some unused symbols from the linker script, and rearranged
the sections so that the file is able to be mapped directly into memory
instead of having each section copied.
When debugging, or in panic callstacks, the BSP idle thread used to be
reported as `_kernel_start`, because it was just the loop at the end of
that assembly function. Now, wrap that loop in a separate symbol called
`bsp_idle` to make it clearer that the cpu is in the idle thread.
Created the framework for using different loadable panic handlers,
loaded by the bootloader. Initial panic handler is panic.serial, which
contains its own serial driver and stacktrace code.
Other related changes:
- Asserts are now based on the NMI handler - panic handlers get
installed as the NMI interrupt handler
- Changed symbol table generation: now use nm's own demangling and
sorting, and include symbol size in the table
- Move the linker script argument out of the kernel target, and into the
kernel's specific module, so that other programs (ie, panic handlers)
can use the kernel target as well
- Some asm changes to boot.s to help GDB see stack frames - but this
might not actually be all that useful
- Renamed user_rsp to just rsp in cpu_state - everything in there is
describing the 'user' state
The kernel's file header has not been verified for a long time. This
change returns file verification to the bootloader to make sure the ELF
loaded in position 0 is actually the kernel.
